The adsorption equilibrium of bovine serum albumin on Q-sepharose, a strong anion exchanger, was studied with batch equilibrium experiments at pH between 4 and 9 and ionic strengths between 5 and 440 mmol/L. Dependence of the adsorption equilibrium on the ionic strength was modeled as an ion exchange reaction. A simplified mechanism of this ion exchange reaction also yielded an expression for the dependence of the equilibrium on the charge of the protein. This model describes the measurements well, using fitted constants with physically realistic values.