--- src/ooxml/java/org/apache/poi/xssf/model/StylesTable.java	(revision 1760296)
+++ src/ooxml/java/org/apache/poi/xssf/model/StylesTable.java	(working copy)
@@ -276,6 +276,10 @@ 
         return numberFormats.get(fmtId);
     }
     
+    /**
+     * Case sensitive search.
+     * Runs in O(n) time. Consider using a bidirectional map for faster reverse lookups.
+     */
     private short getNumberFormatId(String fmt) {
      // Find the key, and return that
         for (Entry<Short,String> numFmt : numberFormats.entrySet()) {
@@ -383,7 +387,7 @@ 
     }
 
     public XSSFFont getFontAt(int idx) {
-        return fonts.get(idx);
+        return fonts.get(idx); // 0-based index
     }
 
     /**
@@ -403,15 +407,15 @@ 
         }
 
         if (idx != -1) {
-            return idx;
+            return idx; // 0-based index
         }
 
         idx = fonts.size();
         fonts.add(font);
-        return idx;
+        return idx; // 0-based index
     }
     public int putFont(XSSFFont font) {
-        return putFont(font, false);
+        return putFont(font, false); // 0-based index
     }
 
     public XSSFCellStyle getStyleAt(int idx) {
@@ -418,7 +422,7 @@ 
         int styleXfId = 0;
 
         // 0 is the empty default
-        if(xfs.get(idx).getXfId() > 0) {
+        if(xfs.get(idx).getXfId() > 0) { // 0-based index
             styleXfId = (int) xfs.get(idx).getXfId();
         }
 
@@ -426,15 +430,19 @@ 
     }
     public int putStyle(XSSFCellStyle style) {
         CTXf mainXF = style.getCoreXf();
+        int idx = xfs.indexOf(mainXF);
 
-        if(! xfs.contains(mainXF)) {
-            xfs.add(mainXF);
+        if (idx != -1) {
+            return idx; // 0-based index
         }
-        return xfs.indexOf(mainXF);
+
+        idx = xfs.size();
+        xfs.add(mainXF);
+        return idx; // 0-based index
     }
 
     public XSSFCellBorder getBorderAt(int idx) {
-        return borders.get(idx);
+        return borders.get(idx); // 0-based index
     }
 
     /**
@@ -447,15 +455,15 @@ 
     public int putBorder(XSSFCellBorder border) {
         int idx = borders.indexOf(border);
         if (idx != -1) {
-            return idx;
+            return idx; // 0-based index
         }
         borders.add(border);
         border.setThemesTable(theme);
-        return borders.size() - 1;
+        return borders.size() - 1; // 0-based index
     }
 
     public XSSFCellFill getFillAt(int idx) {
-        return fills.get(idx);
+        return fills.get(idx); // 0-based index
     }
 
     public List<XSSFCellBorder> getBorders(){
@@ -484,15 +492,15 @@ 
     public int putFill(XSSFCellFill fill) {
         int idx = fills.indexOf(fill);
         if (idx != -1) {
-            return idx;
+            return idx; // 0-based index
         }
         fills.add(fill);
-        return fills.size() - 1;
+        return fills.size() - 1; // 0-based index
     }
 
     @Internal
     public CTXf getCellXfAt(int idx) {
-        return xfs.get(idx);
+        return xfs.get(idx); // 0-based index
     }
     
     /**
@@ -505,18 +513,18 @@ 
     @Internal
     public int putCellXf(CTXf cellXf) {
         xfs.add(cellXf);
-        return xfs.size();
+        return xfs.size(); // 1-based index
     }
     
     @Internal
     public void replaceCellXfAt(int idx, CTXf cellXf) {
-        xfs.set(idx, cellXf);
+        xfs.set(idx, cellXf); // 0-based index
     }
 
     @Internal
     public CTXf getCellStyleXfAt(int idx) {
         try {
-            return styleXfs.get(idx);
+            return styleXfs.get(idx); // 0-based index
         }
         catch (final IndexOutOfBoundsException e) {
             return null;
@@ -534,12 +542,12 @@ 
     public int putCellStyleXf(CTXf cellStyleXf) {
         styleXfs.add(cellStyleXf);
         // TODO: check for duplicate
-        return styleXfs.size();
+        return styleXfs.size(); // 1-based index
     }
     
     @Internal
     protected void replaceCellStyleXfAt(int idx, CTXf cellStyleXf) {
-        styleXfs.set(idx, cellStyleXf);
+        styleXfs.set(idx, cellStyleXf); // 0-based index
     }
 
     /**
@@ -762,7 +770,7 @@ 
 
     @Internal
     public CTDxf getDxfAt(int idx) {
-        return dxfs.get(idx);
+        return dxfs.get(idx); //0-based index
     }
 
     /**
@@ -775,7 +783,7 @@ 
     @Internal
     public int putDxf(CTDxf dxf) {
         this.dxfs.add(dxf);
-        return this.dxfs.size();
+        return this.dxfs.size(); //1-based index
     }
 
     /**
@@ -798,7 +806,7 @@ 
         xf.setBorderId(0);
         xf.setXfId(0);
         int indexXf = putCellXf(xf);
-        return new XSSFCellStyle(indexXf - 1, xfSize - 1, this, theme);
+        return new XSSFCellStyle(indexXf - 1, xfSize - 1, this, theme); //0-based index
     }
 
     /**