Nickel oxide particles were prepared by spray pyrolysis of aqueous solutions of NiSO(4)center dot 6H(2)O and Ni(NO3)(2)center dot 6H(2)O. In spray pyrolysis reactor hollow salt particles initially formed were collapsed by decomposition to reduce their size. For NiSO(4)center dot 6H(2)O less hollowness of the primitive particles and its higher decomposition temperature made the oxide particles highly spherical with very smooth surface. On the other hand the particles prepared from Ni(NO3)(2)center dot 6H(2)O were so hollow and fragile with rough surface since they were formed on the liquid pool of the salt melt. The particle size decreased with the furnace set temperature while increased with the initial salt concentration. Single oxide particle was composed of many small nuclei without sintering whose size varied with the rate of decomposition. The crystallinity of the particles increased with both temperature and the initial salt concentration. Preliminary drying in diffusion dryer fixed the size of the oxide particles from NiSO(4)center dot 6H(2)O at that of the primitive particles, independent of the temperature. However, by the preliminary drying the particles from Ni(NO3)(2)center dot 6H(2)O became more hollow and fragile, whose sizes decreased with the temperature. (c) 2005 Springer Science + Business Media, Inc.