The effect of small additions of MnO (0 to 2 mole %) on sintering of Sm-PbTiO3 ceramics has been investigated. Theoretically dense bodies were achieved at 1150°C by adding 1 mole % MnO, and greater amounts produced a considerable grain growth and density decreasing. The electromechanical behaviour of the sintered ceramics as a function of the MnO content and poling field also was studied, and Kt saturation at relatively low poling fields (50 kV cm-1) only was achieved in samples containing 2 mole % MnO. Although higher Kt values were measured on the denser ceramics a much lower Kt/Kp ratio also was obtained. Sintered ceramics with less dense and larger grain size showed a very small Kp (Kt/Kp ratio higher than 25).