Severe protein fouling is usually encountered during the ultrafiltration of enzymatic hydrolysates from casein on polysulfone membranes. Previous work on the effect of physico-chemical parameters on the flux decline during ultrafiltration suggested that the adsorption of peptides onto the membrane material is responsible for the early stages of fouling and also strongly affect the subsequent evolution of the flux. In the present work, surface energy parameters of polysulfone (PS) and polyethersulfone (PES) membranes were determined from advancing contact angle measurements with three different solvents on hydrated membranes. Changes in membrane surface energetics upon adsorption of components from casein hydrolysates were measured. Adsorption experiments were performed at three pH levels (6,0, 8,0, and 10,0), with and without EDTA. The non-polar contribution to the surface energy was slightly lower for PES than PS. Both materials were highly monopolar, exhibiting a strong electron donor component. The interfacial pressure associated with peptide adsorption was higher for PS than PES, suggesting a higher tendency to peptide adsorption on PS. Interfacial pressure decreased with increasing pH during adsorption on PS, while the reverse trend was observed for PES. The addition of EDTA had very slight effects on the interfacial pressure values of PES, but it reduced that of PS at pH lower than 10,0. Water molecules in the hydration layer showed a high degree of orientation (>85%) for both membranes, suggesting a strong entropic contribution to the adsorption of peptides on PS and PES.