The change in entropy, ΔS, at the first-order magnetoelastic phase transition in Gd5(SixGe1−x)4 alloys for x⩽0.5 has been measured with a high-sensitivity differential scanning calorimeter with built-in magnetic field, H. Scaling of ΔS is achieved by changing the transition temperature, Tt, with x and H from 70 to 310 K. Tt is thus the relevant parameter in determining the giant magnetocaloric effect in these alloys. The calorimetric determination of the change in entropy is also in agreement with the indirect calculation obtained from the magnetization curves measured up to 23 T using both the Clausius–Clapeyron equation and the Maxwell relation. A simple phenomenological model based on the magnetization curves accounts for these results.