The simple model proposed by Kennedy et ar. was modified so as to improve its ability to predict the periodic performance operation of RO systems. The modified model was validated using data fi om the literature as well as data obtained from experiments in this work. The predicted permeate rates compared well with the experimental values for the considered forcing functions: sine and asymmetric square waves. Periodic and steady-state experimental runs were performed on a spiral-wound RO membrane system at three average pressures: 20, 25 and 30 bars. For the case of periodic operation, the operating pressure was varied according to an asymmetric square wave having a period of 5 min. The duration of each run was 30 min. The effect of the time split, gamma, of the forcing wave on the permeation rate as well the salt rejection was investigated. The results showed that periodic operation leads to improvements in the permeation rate. For the tested pressures, the overall maximum water production rates occurred at a time split approximately equal to 0.65. It was also found that cyclic operation may lead to small reductions in the salt passage.