The equiatomic CrMnFeCoNi alloy is one of the most widely studied HEAs and whilst it exhibits many useful properties, the precipitation of sigma phase below 800°C makes it unsuitable for structural applications at elevated temperatures. There is, therefore, an interest in developing non-equiatomic alloys from the CrMnFeCoNi system that suppress sigma formation and offer enhanced mechanical properties. However, effective alloy design requires knowledge of how compositional variations influence phase stability. Here we report experimental studies that establish the effect of Co and Fe on the phase stability in the CrMnFeCoxNi and CrMnFexCoNi systems, where x = 0, 0.5, 1.5, following heat treatments of at least 1000 hours at 500, 700 and 900°C. Both elements stabilise the fcc solid solution phase and their removal leads to the bulk formation of multiple intermetallic phases. Using this data an assessment of the fidelity of current thermodynamic predictions will be made.