The theoretical model proposed for the description of protein adsorption at liquid/fluid interfaces, assumes aggregation of protein molecules in concentrated surface layers. The quasichemical approach is used to describe the equilibrium between monomers and aggregates. Equations derived earlier for surface layers of protein molecules adsorbed in multiple states are generalized for the case of molecular aggregation. The new theoretical model agrees well with experimental results obtained for adsorbed and spread monolayers of bovine serum albumin and beta casein.