Effects of Pb(Mg1/3Ta2/3)O-3 and Pb(Zn1/3Nb2/3)O-3 substitution in the Pb(Mg1/3Nb2/3)O-3-PbTiO3 ceramic system on structure formation, crystallographic aspects, and dielectric properties are discussed. Developed phases in the B-site precursor and perovskite systems were studied by X-ray diffraction, Crystal symmetries and dimensions of the perovskite unit cell of the two systems are compared. Changing rates of the lattice parameter with substituent PbTiO3 concentration in the two modified systems are virtually identical to that of the unmodified Pb(Mg1/3Nb2/3)O-3-PbTiO3 system. Weak-field low-frequency dielectric responses of the ceramics were investigated, The dielectric maximum temperatures of the two perovskite systems shifted almost linearly with compositional change. Dielectric constant spectra at low concentrations of PbTiO3 exhibited typical diffuse phase transition behavior, whereas those at high PbTiO3 concentrations were rather sharp. The phase transition modes reflected on the dielectric spectra were quantitatively analyzed in terms of diffuseness parameters.