Electron spin resonance (ESR) spectroscopy has been used as a method to study membrane fouling during ultrafiltration (UF). Bovine serum albumin (BSA) and polyethylene oxide (PEO), with and without TEMPO (spin probe), solutions in water were used as the feed in the UF experiments. Asymmetric membranes were prepared by phase inversion technique using casting solutions of polyethersulfone (PES) in n-methyl pyrrolidone (NMP). Polyvinyl pyrrolidone (PVP) was added as a nonsolvent additive. The fouling of the membrane was dependent on the size and shape of the macromolecular solute and the operating feed pressure. Tt was also observed that the mobility of the deposited macromolecular solute in the membrane pore became the least in a certain range of pore size of UF membranes and this minimum mobility was also dependent on the operating pressure.