It has been established that two series of mixed crystals are formed in the Cu(HCOO) 2 -Mn(HCOO) 2 -2H 2 O system at 25°C. The thermal decompositions of Cu x Mn 1؊x (HCOO) 2 • 2H 2 O mixed crystals (04x40.5) have been studied. On the basis of the methods used (TG, DTA, and X-ray diffraction analysis), it has been established that homogeneous and heterogeneous oxide phases are obtained depending on the x-values. At about 300°C the formates decompose to Mn 5 O 8 (x ؍ 0); Cu 0.4 Mn 4.6 O 8 (x ؍ 0.08); a mixture of Cu 0.4 Mn 4.6 O 8 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0.08 < x < 0.5), and a Cu 1.5 Mn 1.5 O 4 spinel (x ؍ 0.5). Cu 0.4 Mn 4.6 O 8 (Cu 2؉ 0.4 Mn 2؉ 0.4 Mn 4؉ 3 O 8 ) crystallizes in the monoclinic system with lattice parameters a ؍ 10.39(1) A s , b ؍ 5.729(4) A s , c ؍ 4.862(5) A s , ؍ 109.4°and space group C2/m. At about 600°C the oxides obtained transform into -Mn 2 O 3 (x ؍ 0) and a mixture of -Mn 2 O 3 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0 < x < 0.5). The slowly cooled specimen shows a fine structure in the IR spectrum due to the 1 : 3 ordering of the ions on the B-sublattice, and the cation valences are Cu ؉ [Cu 2؉ 0.5 Mn 4؉ 4.5 ]O 4 . The magnetic measurements show that the oxides obtained are paramagnetic in the temperature range of 25-300°C, and the 1/ versus T curves obey the Curie-Weiss law. The theoretical and experimental magnetic moments of the oxides are given. They coincide well in the case of the homogeneous samples.