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An empirical model modified from the Langmuir isotherm model to account for the effects of ionic strength on the equilibrium characteristics of protein adsorption onto ion exchange/affinity adsorbents has been proposed and tested against experimental and literature data. The equilibrium isotherms for BSA adsorption onto a polystyrenic anion exchanger, Diaion HPA25, were established for five different NaCl concentrations at 25°C, pH 7.0. The apparent Langmuir parameters in the new model (q_m′ and K_d′), which replace the Langmuir parameters (q_m and K_d) in the original Langmuir model, were determined by non-linear curve fitting. The proposed model has been shown to be applicable to various protein/adsorbent systems.