Octahedral cations of Mg and Ta in Pb[(Mg1/3Ta2, (3))(0.8)Tt(0.2)]O-3 were replaced independently as well as simultaneously by Zn and Nb, respectively, to investigate the effects of composition modification on phase formation and dielectric properties. When Mg was replaced by Zn, the perovskite formation yields decreased continuously to 0% with the Zn fraction. In contrast, nearly phase-pure perovskite powders could be prepared when Ta and the Mg-Ta complex were replaced by Nb and Zn-Nb, respectively. Dielectric relaxation with diffuse phase transition was observed at most of the compositions. Dielectric maximum temperatures increased steadily with the substituent fractions, whereas variations of the maximum dielectric constant were dependent heavily on the substituent species.