Comment 10 for bug 961741

GNOME used to have a set of extra compose key sequences. These caused some conflicts with the ones defined in X.org. GNOME also has some software rules to handle compose key sequences. However with the latest version of GNOME 3 in Ubuntu seem to result in wrong characters, e.g. <c> and <single quote opening or acute> result in "c cedilla" instead of "c acute" even compared to GNOME 2.

All additional sequences collected by GNOME over the past years have recently been merged upstream with X.org. From now one, please use only the compose key sequences as defined in:
  http://cgit.freedesktop.org/xorg/lib/libX11/log/nls/en_US.UTF-8/Compose.pre
it is the most extensive set of compose key sequences to date. Ignore the GNOME look aside definitions, see bug reports in GNOME and X.org for more background information.