Abstract Perrhenate (ReO 4−), as a TcO 4− analogue, was incorporated into mixed-anion sodalites from binary solutions containing ReO 4− and a competing anion X n−(Cl−, CO 3 2−, SO 4 2−, MnO 4−, or WO 4 2−). Our objective was to determine the extent of solid solution formation and the dependence of competing ion selectivity on ion size. Using equivalent aqueous concentrations of the anions (ReO 4−/X n− molar ratio= 1: 1), we synthesized mixed-anion sodalites from zeolite and NaOH at 90° C for 96 h. The resulting solids were characterized by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure (XANES) spectroscopy to determine crystal structure, chemical composition, morphology, and rhenium (Re) oxidation state. Rhenium in the solid phase occurred predominately as Re (VII) O 4− in the sodalites, which have a primitive cubic pattern in the space group P 4¯ 3 n. The refined unit-cell parameters of the mixed sodalites ranged from 8.88 to 9.15 Å and showed a linear dependence on the size and mole fraction of the incorporated anion (s). The ReO 4− selectivity, represented by its distribution coefficient (K d), increased in the following order: Cl−< NO 3−< MnO 4− and CO 3 2−< SO 4 2−< WO 4 2− for the monovalent and divalent anions, respectively. The relationship between the ReO 4− distribution coefficient and competing anion size was nonlinear. When the difference in ionic radius (DIR) between ReO 4− and X n−(n= 1 or 2) was greater than~ 12%, then ReO 4− incorporation into sodalite was insignificant. The results imply that anion size is the major factor that determines sodalite anion compositions. Given the similarity in chemical behavior and anion size, ReO 4− serves as a suitable analogue for TcO 4− under oxidizing conditions where both elements are expected to remain as oxyanions in the+ 7 oxidation state.