A methodology for modeling molecular adsorption in multicomponent systems has been developed. The adsorption process is described using the concept of exchange reactions at the solid-liquid interface. The nonideality of the adsorbed phase is represented using the Wilson solution model. For the computation of the equilibrium bulk phase activity coefficients, the nonrandom two-liquid (NRTL) model is used. The model allows for the existence of multiple adsorbed layers at the interface. Binary surface excess data are used to derive equilibrium constants for each exchange reaction. The internal consistency of the methodology is verified by using two separate consistency tests, i.e., the triangular rule for equilibrium constants in ternary systems and the invariance of equilibrium constants with concentration. The model has been applied to a variety of concentrated or dilute systems, including mixtures with liquid-phase immiscibility. In all examined cases, accurate representation of experimental data has been obtained.