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Take a look at this link from MSDN. It may answer your question in part. In the platform support section: Windows Internet Explorer 9 Important Windows Internet Explorer 9 is only supported if you have installed Visual Studio 2010 Service Pack 1 The following scenarios are supported if you have installed Visual Studio 2010 Service Pack ...

This sounds similar to a problem I dealt with recently using watir-webdriver (which, if you're using webdriver will be similar). In my case I was able to resolve/ workaround by sending a click at the element (to select it) before using sendKeys. So I suggest you could try sending a click to the element before using sendKeys. :)