The performance characteristics of two commercially available reverse osmosis (RO) membranes, one cellulose acetate and the other composite polyamide, were investigated with respect to the rejection of different organic compounds, in order to elucidate rejection mechanisms and to investigate correlation with certain solute physical-chemical parameters. Flux and rejection studies were conducted on a series of six alkyl phenols and a series of ten alkyl alcohols. In addition to the RO studies, dissolution/partitioning/sorption tests of phenol in the membrane material were also performed as well as a series of tests to determine the rate of diffusion of phenol in the membranes. It is shown that solute flux in the membrane is a function of effective molecular size of the solute as well as the dissolution and diffusion rates of the solute in the membrane. Solute flux could be correlated with an adjusted total cavity surface area parameter.