X-ray photoelectron spectroscopy (XPS) was used for determination of the effects of chlorhexidine (CHX) solutions (0.2% and 1% solutions of the digluconate salt) on the elemental composition of hydroxyapatite surfaces. So that the nature of the adsorbed species after they were washed with water could be identified, comparisons were made with reference spectra for CHX obtained from a CHX digluconate film and CHX dichloride powder. The XPS results clearly indicated the retention of CHX moieties, which could be ascertained from the spectra by the presence of N and Cl, features unique to CHX. The spectral envelopes were virtually identical to those obtained from the reference spectra. High-resolution C 1s spectra also gave support for the retention of CHX; however, the spectra differed from those of the CHX digluconate film in that no feature attributable to the C-OH of the gluconate anion was present, consistent with the view that the CHX cation remains behind to form an electrostatic bond with the phosphate groups of the hydroxyapatite. The N:Cl ratio for the washed samples was found to be higher than that for the reference samples and may be indicative of partial decomposition of the CHX. Decomposition was also seen to be induced by x-ray exposure. While the high-resolution spectra presented here do not directly address the controversy on the mechanism for the anti-plaque efficacy of CHX, they do provide the necessary basis for the application of XPS to future in vitro studies on the retention of CHX to dental surfaces.