Powders in the Pb(Mg1/3Nb2/3)O3–Pb(Zn1/3Nb2/3)O3 system, with a high perovskite yield, were prepared by a B-site precursor method. PbO was added to pre-reacted B-site components of Mg1−xZnxNb2O6 solid solution, which was further calcined to form a perovskite structure. Perovskite phase contents and lattice parameters were obtained from X-ray analyses. Weak-field low-frequency dielectric constants and losses of sintered pellets were measured as functions of composition, temperature and frequency. Dielectric relaxation behaviours were investigated in terms of diffuseness coefficients. Microstructures were observed to correlate with other characteristics.