A modified mixed oxide synthetic route has been developed for the synthesis of lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃; PMN] and lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃; PFN] powders. The formation of perovskite and pyrochlore phases in the calcined PMN and PFN powders has been investigated as a function of calcination temperature and time by XRD and DTA techniques. The particle size distribution of calcined powders was determined by laser diffraction, with the morphology, phase composition and crystal structure determined via SEM, TEM and EDX techniques. In both cases it has been found that cubic pyrochlore phases in the PbO–Nb₂O₅ system tend to form, as well as the perovskite phase. However, pyrochlore-free PMN and PFN powders were successfully obtained for a calcination temperature of 800°C for 4 and 3 h, respectively, without the introduction of excess PbO and/or MgO.