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.