The magnetic contribution to the martensitic transformation entropy change plays a crucial role in the transition properties of metamagnetic shape memory alloys. This contribution leads to a common dependence of the total entropy change at the structural transformation, ΔStr, on the normalized temperature difference δAtr=(TCA-Ttr)/TCA (where TCA is the Curie temperature of the austenitic phase and Ttr is the transformation temperature) whose main feature is a decrease in ΔStr with increasing δAtr. A phenomenological model based on a Bragg–Williams approximation has been proposed to describe the observed behaviour in terms of the evolution of the magnetic contribution to the total entropy change. The proposed approximation provides a good fitting between predicted and experimental entropy changes obtained for different Ni–Mn-based alloy systems.