Organosilica membranes were prepared by homo-and copolymerization of hydroxymethyl(triethoxy)silane (HMTES) for the first time, and applied to reverse osmosis (RO) to investigate the effects of the hydroxymethyl group. Fourier transform infrared (FT-IR) spectrometry, thermogravimetric analysis (TGA), and water contact angle measurements of the membrane surface indicated that the C-OH group of HMTES underwent a condensation reaction with Si-OH to form an Si-O-C bond during the calcination at 300 °C.Upon RO experiments usinganNaCl aqueous solution, the HMTES membrane exhibited a significant increase of the liquid permeance and a decrease of the NaCl rejection during the measurement, which arose from the hydrolytic cleavage of the Si-O-C bond to form Si-OH and C-OH groups upon contact with water.On the other hand, for a membrane derived from the copolymerization of HMTES and bis(triethoxysilyl)ethane (BTESE) in the weight ratio of 1:1, liquid permeance was increased gradually to reach a steady state of 3.4×10 -13 m 3 /(m 2 •s•Pa), and NaCl rejection was maintained at a