The defect structures occurring in the different phases of CdP2 and ZnP2 are studied by means of electron diffraction and high resolution electron microscopy. The occurrence of numerous planar interfaces in the β‐phase of CdP2 suggests a possible explanation for the series of microphases observed in this type of compound. The absence of thermal dilatation in certain small temperature ranges is explained by assuming that the thermal expansion is in fact compensated in such temperature intervals by the formation of new microphases with a different stacking resulting in smaller lattice parameters.