It is well known that the martensite start temperature in Ni-Ti-based shape memory alloys (SMAs) strongly depends on alloy composition. In the present study we take a new look on this phenomenon. Using differential scanning calorimetry and density functional theory simulations we show that the heat of transformation ΔH decreases as the Ni-concentration increases from 50.0 to 51.2 at.% for binary Ni-Ti. This causes a shift in the Gibbs free enthalpy difference of austenite GA(T) and martensite GM(T), which in turn results in a lower martensite start temperature. In the present work we provide precise reference data sets on the dependence of phase transformation temperatures for stoichiometric and Ni-rich Ni-Ti as well as Ni-Ti-Hf high temperature SMAs. We also discuss possible mechanisms how alloy chemistry governs phase transformation behavior in Ni-rich Ni-Ti-Hf high temperature SMAs.