Uniform spherical Cu2O particles with an average diameter of 0.27 mu m were prepared in large quantities by aging a 0.5 mol dm(-3) CuO suspension containing 0.5 mol dm(-3) N2H4 and 3 wt% deionized gelatin for 3 h at 30 degrees C. Use of amorphous Cu(OH)(2) as a reservoir of copper ions in place of the crystalline CuO yielded polydispersed particles due to the too high solubility of Cu(OH)(2), causing the renucleation of Cu2O during its growth. Gelatin played a decisive role as an inhibitor of the direct attack of N2H4 to CuO surfaces and coagulation of the growing Cu2O particles as well as a pH buffer in producing the uniform particles. The Cu2O particles grew through the surface reaction of the Cu+ ions and/or their complexes formed by the reduction of Cu2+ ions dissolved from the CuO particles. The reducing activity of hydrazine was controlled by regulating the initial pH and by the pH buffer action of gelatin. The dissolution process of Cu2+ ions from the CuO solid was the rate-determining step in the growth of Cu2O particles at pH no less than 9.3, so that the supersaturation was kept low enough to prevent significant renucleation. But, an excessively high pH above ca. 10 led to the formation of metallic Cu particles of a rather broad size distribution due to the too high reducing activity of hydrazine. Under standard conditions, 38% of N2H4 was consumed by the reaction N2H4 + CuO --> 1/2 Cu2O + 1/2 N-2 + NH3 + 1/2 H2O and 15% by (NH4)-H-2 + 4 CuO --> 2 Cu2O + N-2 + 2 H2O.