<p>Ion exchange equilibrium of proteins with different molecular weights and similar isoelectric points has been studied in binary-protein system. Bovine Serum Albumin (BSA) and ferritin have been selected as reference for low and high molecular weight proteins. Uptakes of both proteins in the composite anion-exchanger Q Hyper Z have been studied in simultaneous and sequential ion-exchange batch experiments. In simultaneous ion-exchange experiments, the equilibrium uptake of BSA was strongly affected by the presence of ferritin contrarily to the latter one which was not intensively changed in the presence of BSA. An equilibrium model based on the Steric Mass Action law was able to predict experimental behaviour for BSA ion exchange in single-protein solution, at different salinity. In multicomponent system, ferritin uptake has been well predicted but SMA model was not able to predict BSA behaviour. A selectivity factor between ferritin and BSA, determined by studying the exchange of BSA by ferritin, must be taken into account. The displacement of BSA by ferritin has been highlighted in the sequential sorption experiment and Vroman effect seems take place at the experimental conditions used even if the BSA concentration in solution was too low and cannot be detected.</p>