The phase stability of the &ggr;-cubicLaSe1.47,LaSe1.48andNdSe1.47compounds with theTh3P4type structure, along with their electrical resistivity, Seebeck coefficient, and Hall effect were measured periodically as the materials were annealed at 800 °C or 1000 °C to determine their potential as thermoelectric materials for long term applications at high temperatures. TheLaSexsamples showed the presence of the ternary oxideLa10Se14O(&bgr;-phase). The content of this phase increased with increasing anneal time, whereas theNdSe1.47samples showed no sign of a ternary oxide second phase. The room temperature electrical resistivity and Seebeck coefficient ofLaSexincreased with increased annealing time, while those ofNdSe1.47decreased. The power factors remained relatively stable with annealing time. The high temperature electrical resistivity ofLaSe1.47andNdSe1.47increased with increasing temperature up to 1000 °C, while the Seebeck coefficient became more negative. The power factor, however, peaks at 800 °C forLaSe1.47and at 500 °C forNdSe1.47.Based on this study we find thatLaSexis not a promising high temperature thermoelectric material due to the formation of the &bgr;-phase, whereas theNdSe1.47which does not form the &bgr;-phase oxide is a potential candidate material. ©1997 American Institute of Physics.