Nickel oxides (NiO) nano- and submicron particles were synthesized by a low-pressure spray pyrolysis system. Conversion of micrometer droplets into particles in the gas phase using NiO synthesis as a model was studied. Different types of precursors (aqueous Ni(NO3)(2).6H(2)O, NiCl2, and Ni(HCO2)(2).2H(2)O) and temperatures from 400 to 900 degreesC have great influences on controlling the size and morphology of final particles. The results show that operation of a low-pressure system leads to substantial changes in size and morphology of particles, and that the NiO particles having size around 20 nm can be generated under a limited of conditions. (C) 2003 Elsevier Ltd. All rights reserved.