"The lattice deformations in two doped fluorite systems, (U1-xGdx)O2.00 and (Th1‑xGdx)O2-x/2, have been reassessed by precise X-ray and electron diffraction investigations and the results were interpreted using the Bond Valence Sum (BVS) approach. For the (U1-xGdx)O2.00 system, the experimental findings and theoretical assessment confirm earlier work: the lattice keeps its fluorite structure with a unit cell parameter that contracts linearly with dopant concentration. The lattice contraction in the (Th1‑xGdx)O2-x/2 system has for the first time been analyzed up to the solubility limit. Similar as for (U1-xGdx)O2.00, the (Th1‑xGdx)O2-x/2 solid solution contracts linearly as a function of dopant concentration but additionally, it develops a superstructure which is closely related to the parent fluorite structure. An excess anion bixbyite trial model is proposed to describe this superstructure."