Morphotropic phase boundary (MPB) compositions, separating the rhombohedral and tetragonal phases within the Pb (Mg1/3 Ta2/3) O3 - PbZrO3 - PbTiO3 (PMT-PZ-PT) ternary system, were identified using x-ray diffraction and confirmed by their dielectric and piezoelectric properties. Optimal properties were found for the MPB composition of 0.4PMT-0.2PZ-0.4PT, with a dielectric constant ��r and a piezoelectric coefficient (d33) of the order of 4000 and 580 pC/N, respectively. The planar and thickness electromechanical coupling factors were found to be kp =57.2% and kt =44.4%, with a moderate Curie temperature of 175 ��C. Compositions with increasing PMT content (>0.3) exhibited relaxorlike characteristics, showing dispersive dielectric behavior with respect to frequency, indicative of the decay of the macrodomain state to microdomain state. �� 2009 American Institute of Physics.