The relationship between feed pH and permeate pH in the reverse osmosis (RO) process was investigated in a pH range of 1.6-7.0 when town water was used as feed. Three types of flat-sheet RO membranes with varying isoelectric points in a pH range of 3.2-6.5 were tested in the laboratory. The experimental results showed that for each RO membrane tested, there was a critical feed pH, below which RO permeate pH was higher than feed pH, but above which RO permeate pH was lower than feed pH. The critical feed pH for all membranes was 4.4-4.5, which was independent on the isoelectric point of the RO membrane used. Explanations on the constant critical feed pH are offered. The existence of HCO3- in the feed and its transmission in the RO process may play a key role on the critical feed pH, which has not been reported previously.