As an alternative - I have used the hover pseudoselector for this.
Here's a personal example:
.sidebar ul.sidebuttons li a
{
font: bold italic x-large/1.1 trebuchet ms,verdana,sans-serif;
display: block; /* effect should be in a new box not inline */
text-decoration: none; /* turn off link underlining */
color: yellow;
background: black url(sidebar_off.png) no-repeat center;
padding: 10px 10px 10px 5px;
margin: 0px 0px 20px 0px;
border: white outset;
border-width: 2px 2px 2px 0px;
text-align: right;
text-transform: lowercase;
}
.sidebar ul.sidebuttons li a:hover
{
background: yellow url(sidebar_on.png) no-repeat center;
color: black;
border: black inset;
text-decoration: none;
border-width: 2px 2px 2px 0px;
}
The HTML looks like:
<div class="sidebar">
<ul class="sidebuttons">
<li><a href="somelink.html" title="A linke">Go Somewhere</a></li>
If you're having trouble, try Firefox Web Developer add-on or something similar to check the style information on that part of the page. It might not be triggering what you think it should be triggering.