  The time-differential perturbed-angular-correlation (TDPAC) technique was
applied to the study of the internal electric-field gradient (EFG) in Eu- and
Ho-sesquioxides in their cubic bixbyite phases. The results, as well as
previous characterizations of the EFG at $^{181}$Ta sites in oxides with the
bixbyite structure, were compared to those obtained in experiments using
$^{111}$Cd as probe, and to point-charge model and {\it ab initio} results
calculations for the EFG tensor at impurity sites in binary oxides. These
studies provide quantitative information about electronic processes and the
structural relaxations induced by the presence of impurity probes in the host
lattices, and confirm the existence of nonionic contributions to the EFG in
these systems. Our FP-LAPW calculations show that this nonionic contribution to
the EFG is the dominating one, and that it is originated in the population of
{\it p} states (5{\it p} in the case of Cd, 6{\it p} for Ta).
