In this study, the isothermal sections of the Co-Re-Ta ternary system at 1100, 1200, and 1300 °C have been experimentally investigated by means of electron probe microanalysis and X-ray diffraction. The results indicated the following: (1) The solid solubilities of the λ3, (εCo, Re), χ-Re7Ta3, and bcc-(Ta) phases were large and changed very little from 1100 to 1300 °C; (2) more interestingly, the λ2 phase, with a very limited solubility of Re, was surrounded by the λ3 phase; (3) the solubility of Re for the μ-Co6Ta7 phase increased slowly from 1100 to 1300 °C. These experimental results will be useful for Co-based high-temperature alloys, especially as a supplement for thermodynamic databases.