Introduction of Pb(Mg1/3Nb2/3)O-3 into Pb(Zn1/3Nb2/3)O-3-PbTiO3 was attempted and dielectric properties of the pseudoternary system were investigated. In order to suppress pyrochlore formations, a B-site precursor method was adopted during powder preparation. Weak-field dielectric constants of the system ceramics were measured as a function of frequency. Phase transition modes in the dielectric spectra were further examined and correlated with diffuseness exponents.