I tried this and worked.

		private void Page_Load(object sender, System.EventArgs e)
		{
			Button1.Attributes.Add("onMouseOver",
				"this.style.backgroundColor='red';");
		}

Unlike VB, both C# and JScript are case sensitive. I think it's 'backgroundcolor' in your code. Change it to backgroundColor and test.
Cetin

>When I use the code below in the load event of the webform, nothing happens when the mouse is over the button. The code I have is below.
>
>
>*******Web Form Load Event Code ************
>

>Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
>     'Put user code to initialize the page here
>     Button2.Attributes.Add("OnMouseOver", "changecolor(this,'red');")
>End Sub
>

>
>******** Resulting HTML Code in runtime ***********
>

><input type="submit" name="Button2" value="Button" id="Button2" OnMouseOver="changecolor(this,'red');" style="Z-INDEX: 101; LEFT: 176px; POSITION: absolute; TOP: 136px" />
>

>
>******** JavaScript Function in html code after the ******
>

><script language="javascript">
>    function changecolor(oBtn, sColor)
>      {
>        oBtn.style.backgroundcolor = sColor;
>      }
></script>
>

>
>
>>Or you can do it while building the page on the server via
>>myctl.attributes.add("onmouseover","javascript:dothis()");