Isostructuralism in two series of double complex salts [M(H2O)x(NH3)6-x] [M′(CN)6] (M, M′= Co(III), Cr(III); x = 0, 1, 2) and [M(H2O)x- (NH3)6-x] [CuCl5] (M = Co(III), Cr(III); x = 0, 1, 2) is reported by the use of X-ray powder diffraction methods. Compounds from the first series belong to the space group R 3, while those from the second belong to the Fd3c group. The crystal structure of [Cr(H2O)(NH3)5] [CuCl5], solved by single crystal X-ray diffraction methods, is cubic, space group Fd3c, with a = 22.208(3) Å, V = 10952 Å3, Z = 32, Dx = 1.92 g cm-1, μ = 9.57 cm-1, F(000) = 6336, Mw = 395.98. Disorder of the water molecule of the [Cr(H2O)(NH3)5]3+ cation among the six coordination sites of the octahedrally coordinated Cr(III) ion was found. A possible explanation of the isomorphism between the compounds within each series is proposed. © 1990.