Thin film composite RO membranes were prepared by coating polyamide over two polysulfone membranes having average pore size distributions of 0.07 and 0.15 mu m, respectively. The former TFC membrane designated as Type 1 shows superior salt rejection efficiency over that found with the latter TFC membrane designated as Type 2 (S/R = 96% for Type 1 versus 65% for Type 2). ATR-IR studies suggest a two-fold thicker skin layer in Type 1 membranes due to reduced penetration of polyamide into the pores of polysulfone whereas in the case of the Type 2 membrane the pores are plugged with polyamide but the skin layer is thinner (0.2 mu m for Type 1 versus 0.1 mu m for Type 2), leading to the possibility of higher degree of defects and more porous and, consequently, lower salt rejection efficiency. The higher bulk density of the Type 2 membrane in the IR penetration region accounts for the overall higher intensities of IR signals compared to those observed with Type 1, wherein the IR beam encounters greater degree of voids that yield no signal. (c) 2005 Elsevier B.V. All rights reserved.