<p>Co–Re alloys are a promising class of high-temperature alloys that exhibit a phase transition between a high-temperature fcc phase and a low-temperature hcp phase. To understand the transition between these phases and also the strength of the material, it is important to study the stacking fault energy since a low stacking fault energy facilitates the transformation and deformation by twinning, but also increases creep strength by splitting of partial dislocations. In this paper, the influence of Cr and Re on the phase stability and on the interplanar distance of the crystal is investigated, and the stacking fault energy in the fcc and hcp phase of pure Co and of a Co-18.75Re-25Cr alloy is studied. It is shown that the stacking fault energy is approximately determined by the energy difference between the hcp and fcc phase in all cases. For the alloy, the energy strongly fluctuates due to the formation or breakage of bonds during slip.</p>