The effect of adjusting pH on membrane filterability of activated sludge mixed liquor in membrane bioreactors (MBRs) was studied. The properties of the mixed liquor were analyzed to investigate the mechanisms. The results indicated that moderately adjusting pH was effective to improve the membrane filterability of the mixed liquor. HCl addition provided H+, which reduced the zeta-potential of the colloidal particles. NaOH addition at pH of 8.0-9.0 released some extractable extra-cellular polymeric substances (eEPS) to the supernatant. Re-flocculation occurred. The fouling propensity of the mixed liquor was therefore reduced. Microbial activity was not significantly affected within the pH of 6.0-9.0.