A crystallographic and thermal study of the system Cu_xM_1-x(HCOO)₂·2H₂O (M:Mn, x=0.47; Co, x=0.35; Ni, x=0.37; Cd, x=0.47) has been performed. The compounds, grown at room temperature, crystallize in space group P2₁/c and are isostructural with Cu_0.5Zn_0.5(HCOO)₂·2H₂O (1), with the cations sharing the two special position sites M1 and M2. In all cases, the best refinement was achieved with the copper atoms occupying preferentially the hexaformate-coordinated site M1, while the M²⁺ cations were mainly localized in the M2 sites, in a mixed coordination environment. The compounds present a variety of thermal behaviors, with dehydration taking place at different temperatures and decomposition going from a simple single step process, as in the Zn compound, up to a complex three-stage one as in the Mn and Cd compounds. Decomposition products were identified by X-ray diffraction on quenched samples at the end of each thermal stage. Some discrepancies of the present results with those in the literature are discussed.