Pb(Zn1/3Ta2/3)O-3-PbTiO3 ceramic compositions were modified by the introduction of Nb to the octahedral lattice sites. Resultant tendencies in the perovskite formation and dielectric properties were examined. System powders were prepared using a B-site precursor method. Developed structures and lattice parameters of the system compositions were investigated by powder X-ray diffractometry, from which the parameter of a hypothetical perovskite Pb(Zn1/3Ta2/3) O-3 is proposed. Weak-field low-frequency dielectric responses of the system ceramics were measured. (C) 2003 Kluwer Academic Publishers.