A scheme for classifying the interfacial reactivity of metal–compound semiconductor systems is presented. The scheme uses a simple bulk model to calculate a ‘‘heat of reaction,’’ which includes the effects of metal–semiconductor anion compound formation and metal–semiconductor cation alloying. The scheme is applied to the layered semiconductors GaSe and MoS2, III–V semiconductor InP, and the II–VI semiconductor CdTe, for a wide range of metals. The resulting classification compares favorably with published experimental data on ultrahigh vacuum cleaved surfaces. For the majority of these interfaces, the reactivity classification is the same as that obtained by considering only metal–semiconductor anion compound formation. However, for the combinations Au–InP, Ni–CdTe, Ni–GaSe, Cu–CdTe, and Cu–GaSe, it is only by including alloying that the combination is classified as reactive, in agreement with experimental data.