The site occupancy in the Re–W and Re–Ta sigma phases is studied using a first-principles statistical thermodynamics approach. The factors which govern the stability of the sigma phase were analysed using selected supercells for total energy calculations. Cluster variation calculations in the tetrahedron approximation were performed to study the effect of composition and of finite temperature on the ordering phenomena in the sigma phase. The difference between the site occupancy of both Re–W and Re–Ta sigma phases is discussed.