Water flux and salt rejection rate, which are the two most important parameters in evaluating the performance of a reverse osmosis membrane process, are desirable to be directly related to the membrane properties and operating conditions. However, the membrane transport theories in their general forms are unable to describe the membrane performance satisfactorily. In this study, water and salt fluxes through reverse osmosis membranes were carefully examined with a cross-flow filtration cell under various operating conditions. Experimental results showed that a notable permeate flux was detected when the driving pressure was smaller than the feed osmotic pressure. Water flux increased with the driving pressure nonlinearly before approaching a linear relation with the pressure. In addition, salt transport was highly dependent on both operating pressure and feed salt concentration. A power relationship between salt flux and concentration was correlated well with the experimental data. The equations for water and salt fluxes obtained from this work would provide a facile and accurate means for predicting the membrane performance in design and optimization of reverse osmosis processes.