Abstract The polyol process concerns the preparation of metallic powders by reduction of inorganic compounds in liquid polyols; it essentially applies to cobalt, nickel, copper and precious metals. A general reaction on model has been established with a reaction via the solution: dissolution of the solid precursor; reduction in solution; homogeneous nucleation and growth of the metallic phase from the solution. In the standard conditions the cobalt, nickel and copper particles are isotropic, in the micron range with a narrow size distribution. These characteristics are explained by the homogeneous nucleation model with an effective separation between the nucleation and growth steps. The temperature effect is discussed in relation to this model. Heterogeneous nucleation can be substituted for homogeneous nucleation; it allows, to some extent, a control of the particle size in the submicron range.