This study investigated the effect of acid post-treatment on the filtration performance of organosilica membranes. The membranes were prepared from 1,2-bis(triethoxysilyl)ethane (BTESE) via sol-gel processing with different H2O/BTESE molar ratios. Fourier-transform infrared spectra significant residual ethoxy groups within membranes derived from sols with low H2O/BTESE ratios, after firing at 300 degrees C in N-2. The ethoxy groups within the membranes were hydrolyzed to hydroxide groups by immersing the membranes in hydrochloric acid (HCl). This rendered the membranes more hydrophilic. The hydroxyl groups may have promoted the formation of hydrophilic pores, which would in turn promote water permeation. The HCl-treated membranes showed higher water permeability than the original BTESE-derived membrane. The effect of HCl treatment was more pronounced for the membrane prepared with the lowest H2O/BTESE ratio of 1, resulting in approximately three times higher water permeability than the original membrane. Membranes prepared from sols with low H2O/BTESE ratios showed improved solute rejection and increased water permeability, upon HCl treatment. This was attributed to cross-linking in the organosilica matrix via the condensation of newly-formed hydroxyl groups, that caused the narrowing or blocking of large pores.