Thanks. I finally figured out the problem. The platforms folder wasn't being found because it wasn't included in the search path. I had code that would include the install directory in the search path, using addLibraryPath, but the code was #ifdef'd with the Q_WS_WIN macro. This seems to be Q_OS_WIN in Qt5. So the code wasn't executed. In Chrome, apparently, the current directory is set to the directory of the .dll prior to execution. So the code worked in Chrome. But this doesn't seem to happen in Internet Explorer. At one point I'd run across a potential fix for this problem -- set the environment variable QT_QPA_PLATFORM_PLUGIN_PATH to the platforms folder -- but I think the variable name was misspelled in the forum. So when I tried to set it, it had no effect. BTW, I eventually found the problem by downloading the Qt 5.5.1 source, and using the .pdb files included with my distribution to see what was going on. This was troublesome, but invaluable. So thanks to the Qt developers for making that possible.