A total of 40 at.% Nb was substituted for Ta in the Pb[(Mg,Zn)(1/3)Ta-2/3]O-3 system in order to enhance perovskite development. System powders were prepared by a B-site precursor method. Perovskite formation yields and lattice parameters of the pyrochlore/perovskite structures were determined from X-ray diffraction results. Weak-field low-frequency dielectric responses of the system ceramics were investigated as functions of temperature, composition, and frequency. Phase transition modes reflected in the dielectric constant spectra were further analyzed in terms of diffuseness parameters.