Synthesis and structural features of the relaxor ferroelectric PbMg13Nb23O3, the structural analogue PbMn13Nb23O3 as well as the related phase PbMg14Mn14Nb12O3 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.