Interactions at the bacteria-solid interface in aquatic environments are of crucial importance in biofilm formation processes. The characterization of microbial surfaces properties is therefore a very important step for the understanding of these interactions. In this study, XPS spectra of Bacillus subtilis wet pastes centrifuged from suspensions over a span of pH values were recorded using a fast-freezing procedure. The chemical composition of the bacterial surface was elucidated and the presence of surface sites containing carboxylate, phosphate and amine functional groups was established. It was possible to distinguish between charged and uncharged amine sites on the surface and follow their deprotonation reactions as a function of pH. It is suggested that increased metabolic activity around the physiological pH results in the excretion of proteins and/or peptides which are possibly related to the production of extracellular polymeric substances (EPS). Copyright (C) 2006 John Wiley & Sons, Ltd.