The thermal stability of ceramics based on PMN, PZN and their solid solutions PMN-PT, PZN-PT (PMN = PbMg1/3Nb2/3O3, PZN = PbZn1/3Nb2/3O3, PT = PbTiO3) during sintering is investigated. The present work confirms that the incorporation of a limited excess of MgO in the starting materials is actually quite beneficial for the synthesis of a pure PMN perovskite phase, and that it prevents the formation of a secondary pyrochlore-type phase, after PbO loss during firing. An excess of ZnO is much less efficient for the stabilization of the PZN compound. Examination of the microstructural evolution as well as analysis of second phases in these materials and in their solid solutions formed with lead titanate by wavelength dispersive spectroscopy and/or energy dispersive X-ray spectroscopy techniques, allows a mechanism to be proposed for the stabilization of the PMN-based perovskite phases by an excess of MgO.