A multicomponent transport model is presented to describe the rejection components of electrolyte systems by cellulose acetate reverse osmosis membranes. Comparison of the model with the experimental results obtained with NaCl-CaCl2-H2O solutions shows good agreement. It was observed that the presence of CaCl2can reduce drastically the rejection of NaCl in this multicomponent system and even cause ‘negative rejection’ in certain cases. The results of this study will be helpful in better understanding, predicting and controlling the operation of reverse osmosis desalination plants that process multicomponent feeds where interactions among the electrolytes present can affect the performance of the membranes used