Energetic factors that govern intermixing and alloying at the Cr/Fe(100) interface are discussed and applied quantitatively in a model based on locally composition-dependent, surface enhanced Cr-Fe interactions. The calculations, employing the free-energy concentration expansion method without any adjustable energetic parameters, are in good agreement with scanning tunneling microscopy results for Cr growth on Fe(100), as reported previously by Davies et al. [Phys. Rev. Lett. 76, 4175 (1996)]. Following a moderate, mixing related increase in Cr surface fraction, at ∼1 ML deposited Cr coverage, a sharp transition from a low ∼10% value to ∼30% Cr is predicted. This transition is closely associated with a change of sign in the composition dependent effective pair interactions reported for this system. According to the calculations for deposited coverages >~2 ML, a pure Cr layer is separated at the surface, with 1–2 layer sharp interface and slightly Cr-mixed iron layers underneath. At submonolayer Cr deposited coverages only the latter, diffuse part of the interface prevails. Calculated short-range order pair probabilities at the alloy surface and in the bulk agree with experimental values.