Nonaqueous copper(II)-loaded carboxylate solutions were hydrolyzed directly by water at elevated temperatures to precipitate copper oxides fine particles. Under solvothermal conditions, 170degreesC and 0.8 MPa to 210degreesC and 2.1 MPa for 120 min, a mixture of cupric oxide (CuO) and cuprous oxide (Cu2O) was prepared from the organic starting solution of 0.15 kmol/m(3) copper(II) carboxylate and 0.36 kmol/m(3) free carboxylic acid. When the initial carboxylic acid concentration in the organic starting solution was increased to 1.0 kmol/m(3), thermal treatment of the copper(H) carboxylate solutions at 180-210degreesC resulted in the formation of cuprous oxide alone. The average particle size of the resulting cuprous oxide significantly decreased from 16 to 5.5 mum as the stirring speed of the starting solution was changed from 300 to 100 rpm. The precipitation rate of cuprous oxide in a batch autoclave was markedly dependent on the operating temperature, and the copper(II) in the carboxylate solution was completely precipitated as cuprous oxide within 60 min at 210degreesC.