diff --git a/src/buildHTML.js b/src/buildHTML.js
index 51aece30..299b70e8 100644
--- a/src/buildHTML.js
+++ b/src/buildHTML.js
@@ -539,8 +539,11 @@ groupTypes.xArrow = function(group, options) {
     const arrowShift = -options.fontMetrics().axisHeight +
         0.5 * arrowBody.height;
     // 2 mu kern. Ref: amsmath.dtx: #7\if0#2\else\mkern#2mu\fi
-    const upperShift = -options.fontMetrics().axisHeight -
+    let upperShift = -options.fontMetrics().axisHeight -
         0.5 * arrowBody.height - 0.111;
+    if (group.value.label === "\\xleftequilibrium") {
+        upperShift -= upperGroup.depth;
+    }
 
     // Generate the vlist
     let vlist;
diff --git a/src/functions.js b/src/functions.js
index 6f2a5988..4ac4c8df 100644
--- a/src/functions.js
+++ b/src/functions.js
@@ -198,6 +198,10 @@ defineFunction([
     "\\xrightharpoonup", "\\xleftharpoondown", "\\xleftharpoonup",
     "\\xrightleftharpoons", "\\xleftrightharpoons", "\\xlongequal",
     "\\xtwoheadrightarrow", "\\xtwoheadleftarrow", "\\xtofrom",
+    // The next 3 functions are here to support the mhchem extension.
+    // Direct use of these functions is discouraged and may break someday.
+    "\\xrightleftarrows", "\\xrightequilibrium",
+    "\\xleftequilibrium",
 ], {
     numArgs: 1,
     numOptionalArgs: 1,
diff --git a/src/katex.less b/src/katex.less
index b0458a6c..1b8471a5 100644
--- a/src/katex.less
+++ b/src/katex.less
@@ -492,14 +492,14 @@
     .halfarrow-left {
         position: absolute;
         left: 0;
-        width: 50.1%;
+        width: 50.2%;
         overflow: hidden;
     }
 
     .halfarrow-right {
         position: absolute;
         right: 0;
-        width: 50%;
+        width: 50.2%;
         overflow: hidden;
     }
 
diff --git a/src/stretchy.js b/src/stretchy.js
index d0ec7d07..9f879d0a 100644
--- a/src/stretchy.js
+++ b/src/stretchy.js
@@ -47,6 +47,9 @@ const stretchyCodePoint: {[string]: string} = {
     xtwoheadrightarrow: "\u21a0",
     xlongequal: "=",
     xtofrom: "\u21c4",
+    xrightleftarrows: "\u21c4",
+    xrightequilibrium: "\u21cc",  // Not a perfect match.
+    xleftequilibrium: "\u21cb",   // None better available.
 };
 
 const mathMLnode = function(label: string): mathMLTree.MathNode {
@@ -143,6 +146,16 @@ const katexImagesData: {
     undergroup: [["leftgroupunder", "rightgroupunder"], 0.888, 342],
     xmapsto: [["leftmapsto", "rightarrow"], 1.5, 522],
     xtofrom: [["leftToFrom", "rightToFrom"], 1.75, 528],
+
+    // The next three arrows are from the mhchem package.
+    // In mhchem.sty, min-length is 2.0em. But these arrows might appear in the
+    // document as \xrightarrow or \xrightleftharpoons. Those have
+    // min-length = 1.75em, so we set min-length on these next three to match.
+    xrightleftarrows: [["baraboveleftarrow", "rightarrowabovebar"], 1.75, 667],
+    xrightequilibrium: [["baraboveshortleftharpoon",
+        "rightharpoonaboveshortbar"], 1.75, 716],
+    xleftequilibrium: [["shortbaraboveleftharpoon",
+        "shortrightharpoonabovebar"], 1.75, 716],
 };
 
 const groupLength = function(arg: ParseNode): number {
diff --git a/src/svgGeometry.js b/src/svgGeometry.js
index 72ca2a9e..ed79ca62 100644
--- a/src/svgGeometry.js
+++ b/src/svgGeometry.js
@@ -269,6 +269,50 @@ c-1 5-5 9-11 9h-2L532 67 19 159h-2c-5 0-9-4-11-9l-5-22c-1-6 2-12 8-13z`,
 
     widehat4: `M1181 0h2l1171 296c6 0 10 5 10 11l-2 23c-1 6-5 10
 -11 10h-1L1182 67 15 340h-1c-6 0-10-4-11-10l-2-23c-1-6 4-11 10-11z`,
+
+    // baraboveleftarrow is from glyph U+21C4 in font KaTeX AMS Regular
+    baraboveleftarrow: `M1 500c30.67-18 59-41.833 85-71.5s45-61.17 57-94.5h23
+c15.33 0 23 .33 23 1 0 .67-5.33 12.67-16 36-16.67 34.67-39 67.33-67 98l-10 11
+h39904v40H96l9 10c27.33 30.67 50.67 65 70 103l14 33c0 .67-7.67 1-23 1h-22
+C116.67 596.33 69 540.67 1 500z M96 480 H400000 v40 H96z
+M1 147 H399905 v40  H1z M0 147 H399905 v40  H0z`,
+
+    // ditto rightarrowabovebar
+    rightarrowabovebar: `M400000 167c-70.67 42-118 97.67-142 167h-23c-15.33 0
+-23-.33-23-1 0-1.33 5.33-13.67 16-37 18-35.33 41.33-69 70-101l7-8h-39905
+v-40h39905c-389 0 0 0 0 0l-7-8c-28.67-32-52-65.67-70-101-10.67-23.33-16-35.67
+-16-37 0-.67 7.67-1 23-1h23c11.33 33.33 30 64.833 56 94.5s54.67 53.83 86 72.5z
+M0 147 H399905 v40  H0z M96 480 H400000 v40 H0z M96 480 H400000 v40 H0z`,
+
+    // The next eight paths support reaction arrows from the mhchem package.
+
+    // The short left harpoon has 0.5em (i.e. 500 units) kern on the left end.
+    // Ref from mhchem.sty: \rlap{\raisebox{-.22ex}{$\kern0.5em
+    baraboveshortleftharpoon: `M507,435c-4,4,-6.3,8.7,-7,14c0,5.3,0.7,9,2,11
+c1.3,2,5.3,5.3,12,10c90.7,54,156,130,196,228c3.3,10.7,6.3,16.3,9,17
+c2,0.7,5,1,9,1c0,0,5,0,5,0c10.7,0,16.7,-2,18,-6c2,-2.7,1,-9.7,-3,-21
+c-32,-87.3,-82.7,-157.7,-152,-211c0,0,-3,-3,-3,-3l399351,0l0,-40
+c-398570,0,-399437,0,-399437,0z M593 435 v40 H399500 v-40z
+M0 281 v-40 H399908 v40z M0 281 v-40 H399908 v40z`,
+
+    rightharpoonaboveshortbar: `M0,241 l0,40c399126,0,399993,0,399993,0
+c4.7,-4.7,7,-9.3,7,-14c0,-9.3,-3.7,-15.3,-11,-18c-92.7,-56.7,-159,-133.7,-199,
+-231c-3.3,-9.3,-6,-14.7,-8,-16c-2,-1.3,-7,-2,-15,-2c-10.7,0,-16.7,2,-18,6
+c-2,2.7,-1,9.7,3,21c15.3,42,36.7,81.8,64,119.5c27.3,37.7,58,69.2,92,94.5z
+M0 241 v40 H399908 v-40z M0 475 v-40 H399500 v40z M0 475 v-40 H399500 v40z`,
+
+    shortbaraboveleftharpoon: `M7,435c-4,4,-6.3,8.7,-7,14c0,5.3,0.7,9,2,11
+c1.3,2,5.3,5.3,12,10c90.7,54,156,130,196,228c3.3,10.7,6.3,16.3,9,17c2,0.7,5,1,9,
+1c0,0,5,0,5,0c10.7,0,16.7,-2,18,-6c2,-2.7,1,-9.7,-3,-21c-32,-87.3,-82.7,-157.7,
+-152,-211c0,0,-3,-3,-3,-3l399907,0l0,-40c-399126,0,-399993,0,-399993,0z
+M93 435 v40 H400000 v-40z M500 241 v40 H400000 v-40z M500 241 v40 H400000 v-40z`,
+
+    shortrightharpoonabovebar: `M53,241l0,40c398570,0,399437,0,399437,0
+c4.7,-4.7,7,-9.3,7,-14c0,-9.3,-3.7,-15.3,-11,-18c-92.7,-56.7,-159,-133.7,-199,
+-231c-3.3,-9.3,-6,-14.7,-8,-16c-2,-1.3,-7,-2,-15,-2c-10.7,0,-16.7,2,-18,6
+c-2,2.7,-1,9.7,3,21c15.3,42,36.7,81.8,64,119.5c27.3,37.7,58,69.2,92,94.5z
+M500 241 v40 H399408 v-40z M500 435 v40 H400000 v-40z`,
 };
 
+
 export default {path};
diff --git a/test/screenshotter/images/ExtensibleArrows-chrome.png b/test/screenshotter/images/ExtensibleArrows-chrome.png
index 68928c4c..21883624 100644
Binary files a/test/screenshotter/images/ExtensibleArrows-chrome.png and b/test/screenshotter/images/ExtensibleArrows-chrome.png differ
diff --git a/test/screenshotter/images/ExtensibleArrows-firefox.png b/test/screenshotter/images/ExtensibleArrows-firefox.png
index 503bab7c..8de4d303 100644
Binary files a/test/screenshotter/images/ExtensibleArrows-firefox.png and b/test/screenshotter/images/ExtensibleArrows-firefox.png differ
diff --git a/test/screenshotter/images/LowerAccent-chrome.png b/test/screenshotter/images/LowerAccent-chrome.png
index 1904361e..7fbcbce4 100644
Binary files a/test/screenshotter/images/LowerAccent-chrome.png and b/test/screenshotter/images/LowerAccent-chrome.png differ
diff --git a/test/screenshotter/images/LowerAccent-firefox.png b/test/screenshotter/images/LowerAccent-firefox.png
index a60fa1ab..779ecdb9 100644
Binary files a/test/screenshotter/images/LowerAccent-firefox.png and b/test/screenshotter/images/LowerAccent-firefox.png differ
diff --git a/test/screenshotter/images/ReactionArrows-chrome.png b/test/screenshotter/images/ReactionArrows-chrome.png
new file mode 100644
index 00000000..d95a50b5
Binary files /dev/null and b/test/screenshotter/images/ReactionArrows-chrome.png differ
diff --git a/test/screenshotter/images/ReactionArrows-firefox.png b/test/screenshotter/images/ReactionArrows-firefox.png
new file mode 100644
index 00000000..e467e4b7
Binary files /dev/null and b/test/screenshotter/images/ReactionArrows-firefox.png differ
diff --git a/test/screenshotter/images/StretchyAccent-chrome.png b/test/screenshotter/images/StretchyAccent-chrome.png
index aff80285..75484d67 100644
Binary files a/test/screenshotter/images/StretchyAccent-chrome.png and b/test/screenshotter/images/StretchyAccent-chrome.png differ
diff --git a/test/screenshotter/ss_data.yaml b/test/screenshotter/ss_data.yaml
index 776e2976..bd31967c 100644
--- a/test/screenshotter/ss_data.yaml
+++ b/test/screenshotter/ss_data.yaml
@@ -223,6 +223,11 @@ Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x} \left(\vphanto
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
 Raisebox: \frac{a}{a\raisebox{0.5em}{b}} \cdot \frac{a\raisebox{-0.5em}{b}}{a} \cdot \sqrt{a\raisebox{0.5em}{b}} \cdot \sqrt{a\raisebox{-0.5em}{b}} \cdot \sqrt{a\raisebox{0.5em}{b}\raisebox{-0.5em}{b}}
+ReactionArrows: |
+    \begin{matrix}
+        A \xrightleftarrows{} B \xrightequilibrium{} C \xleftequilibrium{} D \\
+        A \xrightleftarrows{over} B \xrightequilibrium{over} C \xleftequilibrium{over} D
+    \end{matrix}
 RelativeUnits: |
     \begin{array}{ll}
         a\kern1emb^{a\kern1emb^{a\kern1emb}} &