When I try to view a mobi file with Calibre under Kubuntu 9.04, I get no result - no view window, no error message. Nothing happens.

The on-line User Manual says, "For other formats [than LRF] it uses the default operating system application." What would that be for mobi?

There is a bug with the viewer in *buntu that causes it to segfault. It is due to *buntu shipping with PyQt 4.4 and Qt 4.5. As you've found PyQt 4.4 is not 100% compatible with Qt 4.5. There are two solutions.

1) Install PyQt 4.5.

2) Find the main_ui.py file for the viewer, it should be in some place like /usr/lib/python2.6/dist-packages/calibre/gui2/viewer and comment out (put a # at the beginning of the line) the line that looks like one of the following:

self.view.setUrl(QtCore.QUrl("about:blank"))
self.view.setProperty("url", QtCore.QVariant(QtCore.QUrl("about:blank")))

Not knowing what changing the version of PyQt might entail, indeed, not knowing even what it is (I'm a perl fan, so have avoided python...), I deleted the line you suggested.

All works well now.Thanks a million!