Annealing is commonly used in the recrystallization of metamict minerals in an attempt to reconstruct the original structure. Annealing at 750 °C of Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, resulted in the structural transformation C2/m → P21/a, which defines chevkinite stability in different environments. This transformation seems to be a rapid version of a naturally occurring process that possibly involves twinning of the crystals. Nb-rich chevkinite-(Ce) occurs naturally as two polymorphs, one with the C2/m space group and the other with P21/a. The latter is the stable form under ambient conditions. There are some distinct differences in the values of the structural parameters, such as the average M-O distances or site scattering values of particular sites for both space groups, which can be associated with the redistribution of some lighter cations, mainly Mg2+, within the crystal lattice. The use of complementary experimental techniques (electron probe microanalysis, X-ray diffraction, and photoelectron spectroscopy) has delivered information on the structure and transformation of a very complex, highly zoned and partially metamict solid solution. It should be useful in determining the structure of any mineral where cation disorder is present.