Electron spin resonance (ESR) technique was used to study cellulose acetate (CA) membranes for reverse osmosis (RO) and polyethersulfone (PES) membranes for ultrafiltration. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) was used as a spin probe that was brought into the membranes by immersing the membranes into solutions involving TEMPO, or by blending TEMPO into membrane casting solutions. It was found that the ESR signal from TEMPO in the membrane is useful for the choice of polymeric materials to prepare RO membranes. From ESR results it seems that for CA membranes, which are more hydrophilic and swollen by water, channels through the swollen polymer matrix contribute to the transport of water. On the other hand, the transport of water in PES membranes is primarily through pores and the contribution of the little swollen polymer matrix is insignificant.