The Ni-Mo-O alloy powders were electrodeposited from ammonium sulfate containing electrolytes for different Ni/Mo ions concentration ratios. Electrodeposition was investigated by the polarization measurements. The morphology, chemical composition and phase composition of these powders were investigated using SEM, EDS, AAS and XRD analysis. The EDS and AAS analysis showed that the powder composition depends on the Ni/Mo ions concentration ratio, with the Ni/Mo metals ratio in the powders being the same as the one in the solution. The as-deposited alloy powders were nanocrystalline, while after stepwise annealing at 300, 400, 500 and 600 degrees C for 2 h in N-2 atmosphere their crystallinity became more and more pronounced, with the dimension of crystallites increasing with the increase of the annealing temperature. Already after the annealing at 300 degrees C the presence of two phases, MoO3 and NiMoO4, was identified by XRD, while SEM analysis showed that the surfaces of powder particles were partially recrystallized. With the increase of the annealing temperature the amount of the NiMoO4 was found to increase in all powders. For the powder electrodeposited from the solution with the highest Ni/Mo ratio (1/0.3) NiMoO4 phase was detected together with a small amount of Ni4Mo phase, indicating phase transition of MoO3 into NiMoO4 (and probably Ni4Mo) at the temperature of 600 degrees C. (c) 2008 Elsevier Ltd. All rights reserved.