X-ray photoemission electron microscopy (XPEEM) using synchrotron radiation illumination has been used to study the adsorption of human serum albumin (HSA) onto a phase segregated polystyrene/polymethyl-methacrylate (PS/PMMA) blend surface from solutions of five different pH values. The absolute coverage of albumin on each of three chemically distinct components of the surface, PS domains, PMMA domains, and the interface between the domains, was determined from a quantitative analysis of C is image sequences. At all pH values, the preferred adsorption site is the interface. At neutral pH (7.0), albumin showed a slight preference for PS regions relative to PMMA. At strongly acidic pH (2.0) and strongly basic pH (10.0), similar amounts of albumin adsorb on the PS and PMMA regions. However, at pH 4.0, the amount of albumin adsorbed on PMMA domains is similar to-1.6 times greater than that on PS domains, while at pH 8.6 the amount of albumin adsorbed on PMMA is one-half that adsorbed on PS domains. The pH dependence of the site preference is rationalized in terms of the known changes of albumin conformation with pH [Peters, T., Jr. All About Albumin: Biochemistry, Genetics, and Medical Applications; Academic Press: New York, 1995]. We infer from our results that the site preference of albumin adsorption on PS/PMMA blends is related mainly to changes in hydrophobic interactions, which are driven by pH-dependent electrostatic effects, that is, changes to the protein surface structure as the charge on the protein changes. The results provide insight into changes in the secondary structure of albumin in acid and basic media.