Carbon dioxide plasmas were used to modify hydrophobic polysulfone ultrafiltration membranes to create hydrophilic surfaces throughout the membrane structure. The water contact angle of the upstream side of the membrane (facing the plasma) decreased to zero after treatment and did not change even after several months of aging. The water contact angle of the downstream side decreased with increasing CO2 plasma treatment time and became zero for treatment times greater than or equal to 1 min (P = 10 W). Functional groups introduced by CO2 plasma treatment were examined using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). For the treated membranes, the atomic concentration of oxygen increased dramatically and small amounts of nitrogen incorporation were also observed. Membrane performance was tested with water flux measurements as well as protein fouling studies. For treated membranes, the water flux recovery measured after protein fouling was significantly higher than that for control membranes, with nearly 100% recovery after gentle cleaning in water. Moreover, the amount of protein adsorption decreased by over 75% for the treated membranes compared to control membranes. This suggests the protein fouling layer is essentially completely reversible on the CO2 plasma treated membranes.