Orthopyroxenes are important rock-forming minerals. They are solid solutions mainly between an Mg and a Fe endmember. Their thermodynamic properties are needed to model the P–T–X paths of igneous and metamorphic rocks. Samples along this Fe–Mg binary were synthesized at 800 °C/1.5 GPa, and their heat capacity was measured using calorimetric methods. From these data, the entropies along the solid solution could be derived, and it was found that this solution behaves thermodynamically ideally.