This study investigated the thermal behaviour of the fcc ↔ hcp martensitic transformation of Co–Ni alloys. The transformation was found to be incomplete in both directions upon cooling and heating. Upon thermal cycling, the transformation, as detected by differential scanning calorimetry, diminished rapidly, indicating that the transformation was highly irreversible. Optical microscopic observation revealed that surface reliefs appeared on pre-polished surfaces upon forward transformation, but did not disappear upon the reverse transformation, implying that the transformation also exhibited mechanical irreversibility. The irreversibility is explained on the basis of multiple choice of crystalline shuffling along for the fcc ↔ hcp transformation. These observations suggest that the fcc ↔ hcp martensitic transformation system in Co–Ni is not suitable for thermomechanical shape memory effect, which requires high reversibility of the transformation both crystallographically and mechanically. It is regarded that this rule is generic and applicable to other fcc ↔ hcp transformation systems.