An infrared photothermal deflection spectroscopy system has been developed using infrared tunable diode lasers in the wavelength region from 8 to 16 mu m. This system allows the collection of in situ optical absorption spectra of species at electrode/electrolyte interfaces in an electrochemical cell. The infrared spectra of LiOH and Li2CO3 formed on the surface of a lithium foil were recorded. The infrared photothermal deflection spectroscopy technique has been used to monitor the lithium electrode surface in an operating cell, Li/P(EO)(2)LiN(CF3O2)(2)/Ni. The in situ spectra (in the 800-1000 cm(-1) region) show that the film formation at Li/polymer electrolyte interfaces is dominated by LiOH, although there also appears to be organic species in the surface film.