The equiatomic CrMnFeCoNi alloy exhibits many desirable properties, but its susceptibility to the formation of embrittling intermetallic phases, makes it unsuitable for structural applications at elevated temperatures. As a result, there is an interest in developing alternative alloys from the CrMnFeCoNi system which avoid this limitation. Here we present a detailed study of phase stability in a CrMnFexCoNi series of alloys, where x = 0, 0.5, 1.5 (in atomic ratio), following long-duration heat treatments of 1000 h at 900 and 700˚C, and up to 5000 h at 500˚C. Each alloy was single phase fcc following homogenisation. After exposure at 900˚C, large σ phase precipitates were present in the CrMnCoNi alloy, but alloys containing ≥ 0.5 Fe remained single phase fcc. At 700˚C, the alloys investigated all contained the σ phase, Cr-bcc precipitates were also present in the CrMnCoNi and CrMnFe0.5CoNi alloys and Cr carbide precipitates featured in the CrMnFe1.5CoNi alloy. Heat-treatment of the CrMnCoNi alloy at 500˚C caused a partial bulk decomposition of the fcc matrix, which produced a fine-scale intergrowth of phases: σ, NiMn-L10, Cr-bcc and a secondary solute-depleted fcc phase. In the alloy containing 0.5 Fe, cellular regions consisting of a NiMn-L10, Cr-bcc and solute-depleted matrix phase, developed along the grain boundaries. NiMn-L10 and Cr-rich precipitates also formed on grain boundaries in the 1.5 Fe alloy. From these experimental observations, it was clearly established that Fe stabilises the fcc matrix relative to the σ and bcc phases.