In situ infrared spectroscopy has been used to show that aqueous cadmium sulfide photocorrosion leads to solution sulfate via an adsorbed thiosulfate intermediate. Infrared spectra were recorded from thin films of colloidal cadmium sulfide deposited on zinc selenide internal reflection elements and in contact with aqueous solutions. Infrared absorptions at 1005 and 1152 cm(-1) have been ascribed to adsorbed thiosulfate by comparisons with the corresponding absorptions in solution. Simultaneous in situ photolysis and infrared spectroscopy showed that adsorbed thiosulfate was initially formed but gradually converted to sulfate. Adsorption isotherm data, derived from absorbances in the infrared spectra, showed that thiosulfate is strongly adsorbed and that sulfate is weakly adsorbed to cadmium sulfide. This accounts for the lack of detection of thiosulfate in solution when photocorrosion occurs.