The particle size distributions of cobalt powders prepared by reactive deposition were examined and correlated with the deposition condition. In reactive deposition, a Co (OH)2 colloid layer was formed at the electrode surface upon reduction of dissolved oxygen. The colloid layer accelerated the decrease in interfacial Co2+ concentration and inhibited crystal growth of the metal, and was therefore instrumental in the formation of fine cobalt particles. The powders from reactive deposition were statistically a factor of two finer and of more uniform size distribution than those from normal electrodeposition. It was found that both the reactive deposition of cobalt powders and the reactive deposition of porous cobalt structures were driven by the same kinetic factors, namely the requirement of maintaining an effective presence of the Co(OH)2 colloid layer at the electrode surface. Hence a similar dependence on deposition conditions for both processes was obtained.