The statistical thermodynamics of molecules with multiple adsorption states has been studied using a lattice model. The thermodynamic functions, calculated for a system characterized by two adsorption states, were applied to describe the adsorption of antifreeze proteins onto an ice crystal. The theoretical formalism reproduces the classical Langmuir equation (one-state limit), leads to the exact statistical thermodynamics of molecules adsorbed in one dimension with n different adsorption states, and seems to be a promising way toward a more accurate description of the adsorption thermodynamics of structurally diverse proteins. (C) 2012 Elsevier B. V. All rights reserved.