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In the present work we explain the concentration dependence of the martensite start temperature (M_S) in Ni–Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of M_S on alloy composition in binary Ni–Ti, ternary Ni–Ti–X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni–Ti–Cu–Y (Y = Co, Pd) SMAs. For binary Ni–Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation Δ H decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite G_A(T) and martensite G_M(T), which in turn results in a lower M_S temperature. Our DFT results suggest that the strong decrease of Δ H is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B190. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni–Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease.