Perovskite phase formation and dielectric properties of the Pb(Fe2/3W1/3)O-3-Pb(Mg1/3Nb2/3)O-3 system with substitution of Pb(Fe1/2Nb1/2)O-3 were studied. Ceramic powders were prepared by a B-site precursor method to enhance the perovskite formation yields. Lattice parameter changes were interpreted in terms of B-site cation stoichiometries and sizes. Weak-field low-frequency dielectric characteristics were investigated in order to examine the effect of composition modification on Curie temperatures and maximum dielectric constants.