[en] The solid state interaction of the Zn_1-xCo_xO nominal system is investigated by means of diffusion couples and analysis of co-precipitated samples. The formation of a homogeneous Co:ZnO solid solution is found to be determined by the crystal structure from which Co^II ions diffuse into the wurtzite lattice. No diffusion is observed whenever the CoO rock-salt structure is formed from the Co^II precursor. On the contrary, the diffusion from the Co₃O₄ spinel phase is feasible but has a limited temperature range defined by the reduction at a high temperature of Co^III-Co^II, since this process again leads to the formation of the rock-salt structure. However, when using a highly reactive and homogeneous co-precipitated starting powder, neither the spinel phase nor the rock-salt structure is formed, and a Co^II:ZnO solid solution is obtained, which remains stable up to high temperatures. - Graphical abstract: Maximum diffusion distance for the ZnO-CoO_x couple as a function of temperature. Dashed gray lines represent the temperature values at which the transformations between CoO and Co₃O₄ compounds take place