1. The particle size distribution and specific surface of cobalt powder obtained by the cathodic reduction of basic cobalt carbonate have been determined. As the cobalt content of the powder rises from 90 to 97%, the specific surface of the latter changes from 26.2 to 18.4 m2/g. With increasing degree of oxidation of the powder, the amount of fine fractions decreases. However, the particle size nonuniformity of the powder, as revealed by sieve analysis, is only apparent. The coarse granules consist of finer particles of 0.5–1μ size. 2. The atmospheric corrosion of the cobalt powder was studied both in the presence of sulfur dioxide and without it. It was established that sulfur dioxide (0.1 and 1 vol.%) lowers the critical humidity value from 75–80 to 20%. The corrosive influence of sulfur dioxide becomes intensified with increasing relative humidity of the atmosphere, as well as with growing degree of oxidation and specific surface of the powder. 3. In the presence of sulfur dioxide, the corrosion of the powder as a function of time obeys a parabolic law, which points to the formation of protective films consisting of corrosion products on the powder particle surface. 4. The influence of sulfur dioxide on the corrosion of cobalt powder and powders of other iron-group metals appears to consist both in accelerating the cathodic process as a result of the reduction of tetravalent sulfur on the cathodic portions of the metal being attacked and in oxidizing SO2 to sulfur trioxide with subsequent formation of sulfuric acid, which destroys the protective films on the metal surface.