Ceramic powders of Pb(Zn1/3B"(2/3))O-3 (where B" is Nb or Ta ions) complex-perovskite compositions, with gradual replacements of Pb by Ba, were prepared by using a B-site precursor method. Phases developed in both systems were identified by X-ray diffraction and perovskite contents were evaluated as a function of the amount of Ba substitution. Variations of lattice parameters of the pyrochlore and perovskite structures with composition modifications are interpreted in terms of the 6- and 12-fold cation sizes. Weak-field dielectric properties of the two systems in the low-frequency range are compared.