The complexes [Co (NH3)(6)][Ir (C2O4)(3)] and [Ir(NH3)(6)][Co (C2O4)(3)center dot H2O have previously been synthesized and their thermal properties studied. The [Ir(NH3)(6)][Ir(C2O4)(3)] and [Co(NH3)(6)][Co(C2O4)(3)]center dot 3H(2)O complexes considered here are the end members in a series of possible isostructural solid solutions based on the complex salts in the Co-Ir system. Their crystal structures and thermal properties are described in detail, including temperature-dependent in situ X-ray diffraction. During the thermolysis of these compounds, layered metal nanoparticles are formed. Close attention is paid to the details of the [Co(NH3)(6)][Ir(C2O4)(3)] synthesis. It has been shown that the formation of this complex salt is temperature dependent; upon heating, a new phase of the K-3[Co(NH3)(6)][Ir(C2O4)(3)](2)center dot 6H(2)O salt is formed, which incorporates the initial iridium compound into the crystal structure of the double complex salt. The target [Co(NH3)(6)][Ir(C2O4)(3)] product is obtained if the synthesis is carried out at room temperature.