The phase stability of the γ-cubic LaSe1.47, LaSe1.48 and NdSe1.47 compounds with the Th3P4 type 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. The LaSex samples showed the presence of the ternary oxide La10Se14O (β-phase). The content of this phase increased with increasing anneal time, whereas the NdSe1.47 samples showed no sign of a ternary oxide second phase. The room temperature electrical resistivity and Seebeck coefficient of LaSex increased with increased annealing time, while those of NdSe1.47 decreased. The power factors remained relatively stable with annealing time. The high temperature electrical resistivity of LaSe1.47 and NdSe1.47 increased with increasing temperature up to 1000 °C, while the Seebeck coefficient became more negative. The power factor, however, peaks at 800 °C for LaSe1.47 and at 500 °C for NdSe1.47. Based on this study we find that LaSex is not a promising high temperature thermoelectric material due to the formation of the β-phase, whereas the NdSe1.47 which does not form the β-phase oxide is a potential candidate material. © 1997 American Institute of Physics.