Internal surface coverage of protein in membranes during the initial phase of ultrafiltration was analyzed. Bovine serum albumin (BSA), in solutions of pH 5 and 7 with 0.05 and 0.15 M NaCl, were ultrafiltered with 100,000-MWCO polysulfone membranes. Mass uptake of the membranes was determined using electron paramagnetic resonance spectroscopy for separate membranes for fractions of an hour for up to 4 h. The resulting data were compared with models for surface coverage. The results showed a significant amount of protein loading exists in a very short time of exposure during ultrafiltration. The amount of material found in this period was not a function of solution properties; however, solution properties did affect the resistance per mass for adsorbed species when the pH was near the isoelectric point of BSA. The modeling comparison implied that the entrained solute from pH 7 appeared to behave as though it was lodged in the ultrathin skin area of the membrane. Solute in the membrane at pH 5 and low ionic strength appeared to be lodged and adsorbed throughout the membrane substructure.