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A catalyst precursor with highly developed specific surface area of 10 m2/g and a pore volume of 0.02 cm3/g is synthesized. The peculiarities of the system studied related to the structure of the copper oxalate, crystallizing as an anhydrous salt with “zeolitic type” bonded water, its content varying between 0 and 1, are pointed out. The thermal decomposition is followed by investigating the magnetic properties in situ. The results are complementary to the information obtained by DTA/TG studies. The performance of magnetic measurements and the calculation of the magnetic moment μeff in the range from −100 to 300°C allow a conclusion to be drawn concerning the coordination of the Cu(II) ions and the change in the oxidation state. In the starting oxalate, Cu(II) is in a tetrahedral-like coordination, which is a result of the strong tetragonal deformation of the octahedral field and of the stronger tendency of the oxalate ion to rotate around the C–C bond axis. The dehydration process does not affect the XRD results, but changes the temperature dependence of μeff due to the change in the Cu(II) coordination. The μeff values during the decomposition process suggest that the proportion Cu(II)-Cu(I) could be varied in the final product by varying the temperature range. By isothermal annealing at 300°C for 1 h, an oxide product containing Cu(II)-Cu(I) is synthesized and characterized. The solid phase products corresponding to the separate parts of the DTA/TG curves are: [Cu] → Cu + Cu2O (185–300°C), 0.5Cu2O + 2CuO (300–345°C), 3CuO (345–400°C).