Solid solutions in the lead-based relaxer system Pb(Mg1/3Nb2/3)O-3-PbTiO3 were modified by minor substitutions of Ba in the Pb-site of the perovskite lattice. The modified compositions were calcined at 830 degrees C for 3 h to yield fine-grained, single-phase perovskite materials. A small amount of excess MgO(0.05 wt %), which mostly served as a sintering aid, was added to the calcined batches and the resulting mixtures were sintered at temperatures between 1150 and 1250 degrees C for periods ranging from 3 to 5 h. The substitutions of BaO for PbO in the perovskite solid solution lattice caused a progressive lowering of the Curie point with increasing BaO content. On average, the Cu rie point decreased by about 10 degrees C for each mole of BaO substituted for PbO. Among the various Ba-substituted solid solutions studied, the one with a nominal composition Pb0.99Ba0.01[(Mg1/3Nb2/3)(0.9)Ti-0.1]O-3, which has a Curie point located near 28 degrees C, exhibited excellent dielectric properties. On sintering at 1250 degrees C for 3 h, this composition yielded a density near 96% of the theoretical density. The peak dielectric constant of the composition at 1 KHz was slightly higher than 22 000, and the corresponding tan delta value was 1.5% with a specific resistivity of 2.5 x 10(12) Ohm cm(-1).