NiO-YSZ (YSZ: Y2O3-stabilized ZrO2) composite particles for a Ni-YSZ cermet anode in solid oxide fuel cells (SOFCs) were synthesized via spray pyrolysis (SP). The formation mechanism of the composite particles by this process was analyzed. The internal microstructure of the particles synthesized during SP processing was observed at each heating temperature of 200, 300, 400 and 1000 degreesC, and then the formation mechanism of the composite structure was discussed. As a result, it was found that NiO-YSZ composite particles were formed through the following steps. Firstly, during the evaporation stage up to 200 degreesC, a filled particle with Ni(CH3COO)(2) and YSZ fine grains were formed from the atomized droplet containing Ni ion and dispersed YSZ sol by volume precipitation. Secondly, during the continuous thermolysis stage up to 400 degreesC, YSZ grains were formed and moved to the surface of the composite particle by the outgas and the oxidation of Ni(CH3COO)(2). Finally, the NiO-YSZ composite particle that has NiO grains uniformly covered with fine YSZ grains was formed after the final sintering stage up to 1000 degreesC. (C) 2003 Elsevier Science B.V All rights reserved.