Equilibrium grain boundary segregation levels have been measured on binary and ternary iron alloys with tin and sulphur over a wide temperature range, by means of two complementary techniques: interfacial energy measurements and Auger electron spectroscopy. Segregation levels derived from the Gibbs adsorption theorem applied to solid/solid interfaces are fully consistent with spectroscopy measurements. It is demonstrated that at lower temperatures, equilibrium multilayer segregation exists. The observations are treated in terms of a general theory of grain boundary adsorption, which is the analogue of the B.E.T. (Brunauer, Emmett & Teller 1938) multilayer gas adsorption theory for free surfaces, and which allows for both submonolayer and multilayer segregation at grain boundaries. Furthermore, this treatment provides a physical basis for an earlier empirical correlation between interfacial solute activity and the solid solubility limit.