Abstract This paper explores the role of association on the adsorption isotherms of β-lactoglobulin A on a weakly hydrophobic stationary phase at 4°C and mobile phases of 0.85 M and 1 M ammonium sulfate, pH 4.5. The isotherms, obtained by frontal analysis, show an S-shape and the corresponding Scatchard plots indicate positive cooperativity. The slopes and intercepts of the Scatchard plots at low solute concentration are analyzed in terms of two species —a protomer and a higher order stronger adsorbing species. An explicit equation of the isotherm is developed based on this model, and this expression is shown to reproduce the isotherm shape using the appropriate derived parameters. It is further shown from this equation that a Langmuir-shaped adsorption isotherm can be obtained if the higher order associate or aggregate binds weaker to the support than the protomer. These results indicate that protein—protein interactions and the formation of associates can play a significant role on the shape of the isotherm and ultimately on the behavior of the species in preparative scale chromatography.