Abstract Cobalt ammine sulphate solutions are reduced by gaseous hydrogen at elevated temperatures and pressures to metallic cobalt powder. The reduction is a catalytic reaction. In the refinery, the cobalt is produced in a cycle comprising a nucleation reduction with a suitable catalyst and densification reductions in which the produced cobalt powder is used as a catalyst. The kinetics of the nucleation and of the densification reductions were studied as a function of catalyst concentration, reduction conditions and solution compositions. The physical properties of the cobalt powder are controlled by the reduction procedure and addition of surface active agents.