Powders of a Pb[(Mg1/3Ta2/3)(0.8)(Zn1/3Ta2/3)(0.2)]O-3 system with Nb substitution for Ta were prepared using a B-site precursor method. Phase formation yields as well as lattice parameters of the perovskite structure in the system compositions were determined by X-ray diffraction. Dielectric responses of the ceramics were monitored at weak-field low-frequency conditions. The dielectric constant spectra were further analyzed for diffuseness characteristics of the phase transition mode. Microstructures of the sintered specimens were also examined.