It is demonstrated that attenuated total reflection infrared (ATR-IR) spectroscopy can be effectively used for in situ investigation of supported catalyst-liquid interfaces. Thin (ca. 10 mum) films of a 5 wt % Pt/gamma-Al2O3 catalyst were deposited onto the surface of a germanium waveguide. The very small support particles (average size, 37 nm) form a stable film that does not peel from the waveguide when exposed to flowing liquid. The catalyst film is thick enough so that essentially all of the IR electric field is attenuated within the film thus decreasing bulk liquid signals. The applicability of this technique for studying adsorption and reaction of molecules on supported catalysts has been tested using several probe molecules (carbon monoxide, formaldehyde, ethanol, butyronitrile) and solvents (water, ethanol, hexane). Examination of adsorption of CO from aqueous and ethanolic solutions reveals that CO resides in both atop and bridged configurations on the catalyst surface in both solvents. A 10-fold decrease in the oxidation rate of adsorbed CO in ethanol was observed. This is attributed both to the lower solubility of O-2 in ethanol compared to water and the likely presence of trace ethanol dissociation products that may block O(2)2 adsorption. The dissociation of formaldehyde and ethanol in water was studied by following the formation of adsorbed CO. The extent of dissociation appeared considerably larger for formaldehyde than for ethanol as determined by comparing absorption intensities and peak frequencies. Finally, adsorption of butyronitrile from hexane was examined with a view toward extending this approach to the study of more complex systems. Butyronitrile was found to adsorb on the catalyst by sigma-bonding of the CN group with the platinum. The prospects of using this approach to examine solid-catalyzed liquid-phase reactions are discussed in light of these findings.