Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO2(B), Cr2O3 were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline gamma-Mn2O3 was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single crystal nickel nanowhiskers were synthesized by direct reduction without postannealing process. All the products were characterized on their structure and micro-morphology. The reaction details and features were described and discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.