A synchrotron-based x-ray photoemission electron microscope (X-PEEM) was used to investigate the coadsorption of a mixture of human albumin serum and SUB-6, a synthetic antimicrobial peptide, to a phase-segregated polystyrene/poly(methyl methacrylate) (PMMA) substrate at varying concentrations and pH. The authors show that X-PEEM could detect the peptide adsorbed from solution at concentrations as low as 5.5 x 10(-9)M and could differentiate the four components via near-edge x-ray absorption fine structure spectromicroscopy. At neutral pH the SUB-6 peptide adsorbed preferentially to PMMA. At a pH of 11.8 where the charge on the peptide was neutralized, there was a more balanced adsorption of both species on the PMMA domains. The authors interpret these observations as indicative of the formation of an electrostatic complex between positive peptide and negative protein at pH of 7.0. This solution complex had an adsorption behavior that depended on the polarity of the substrate domains, and favored adsorption to the electronegative PMMA regions. At a pH of 11.8 the complex formation was suppressed and a more competitive adsorption process was observed.