The thermodynamic stability ’- Co3(Al,W) phase (L12) in the Co-Al-W ternary system at 900 °C was investigated through microstructure and EPMA analysis on a heat-treated bulk alloy. To promote microstructural evolution, the bulk alloy was cold rolled before heat treatment. By heating at 900 °C, the ’ phase was formed discontinuously in contact with the -Co (A1) phase. With increasing heat treatment time, however, the fraction of ’ phase decreased while that of , CoAl (B2) and Co3W (D019) phases increased. These results are consistent with our previous work with a diffusion-couple method, indicating that the ’ phase is metastable, and the three phases of, CoAl and Co3W are thermodynamically stable at 900 °C.