Synthesis and structural features of the relaxer ferroelectric PbMg1/3Nb2/3O3, the structural analogue PbMn1/3Nb2/3O3 as well as the related phase PbMg1/4Mn1/4Nb1/2O3 are described. It is shown that pure phases are only obtained when accurate experimental conditions and appropriate precursors are used. The magnetic and dielectric properties are characterized. Investigations on the thermochemical reactivity of these phases reveal that metal selective reduction/reoxidation processes, as well as phase segregations, take place.