Infrared (IR) spectroscopy of thin particle films, deposited on an attenuated total reflection (ATR) crystal and immersed in solution, has high potential for investigating the chemistry of solid-solution interfaces. Phenomena that can be studied include pH-dependent adsorption, photosensitization, photocatalysis, and biocompatibility of prosthetic materials. With films a few micrometers thick, the penetration depth of the evanescent IR wave is sufficient to monitor surface reactions throughout the high surface area particle film.