AbstractIllustrations of the structure of phase boundaries involved in solid‐state reactions in ceramic oxides and of the formation of grain boundaries by the movement of phase boundaries are presented. Examples of experimental studies have been chosen to emphasise how both the cations and anions can influence the structure of the phase boundaries and the manner in which the reaction proceeds. In the oxide systems discussed here, the phase boundaries tend to facet parallel to certain planes even in the case where the anion sublattice is effectively undisturbed. This observation emphasises that, although the anion sublattice often determines the interface structure, the influence of the cations on the selection of the boundary plane cannot be neglected. The discussion is focused here on the growth of spinel in three different oxide matrices, namely in a wustite phase (NiO), a rhombohedral sesquioxide (Al2O3) and an olivine ((FeMg)2SiO4).