Abstract: X-ray Photoelctron spectroscopy (XPS) is used to study BSA adsorption on polysulfone (PS) membrane in static and dynamic conditions. The atomic compositions from XPS measurement show that the adsorption ratio, which is defined as the number of made of BSA over that of PS in the test scope, increases with the adsorption time, and dynamic adsorption is more severe than static adsorption at the same adsorption time. The fitting spectra indicate that there are chemical interactions with electron transfer between BSA and PS membrane. Further analysis shows that O 2 dissolved in BSA solution also participates in chemical interaction. These results can explain why the fouling of PS membrane by BSA solution is serious.