A series of four isostructural double complex salts are prepared on the basis of [M1(NH3)6]3+ cations and [M2(C2O4)3]3– anions, where M1 = Co, Ir, M2 = Fe, Cr. The salts are crystallized in a hexagonal space group P3̄c1. According to thermal analysis data, thermal stability in an atmosphere of argon (starting temperature of the decomposition of dehydrated products) of studied compounds depends on the nature of the complex tris-oxalate anion and increases in the series [M1(NH3)6][Co(C2O4)3] < [M1(NH3)6][Fe(C2O4)3] < [M1(NH3)6][Cr(C2O4)3] < [M1(NH3)6][Ir(C2O4)3] (M1 = Co, Ir). Hexammineiridium(III) salts are more stable than cobalt(III) salts with the same anion. Thermal decomposition of [Co(NH3)6][Fe(C2O4)3]·3H2O and [Ir(NH3)6][Fe(C2O4)3]·3H2O salts in an atmosphere of hydrogen leads to the formation of solid solutions Co0.5Fe0.5 and Ir0.5Fe0.5, respectively.