Photoinduced polymerization of styrene monomer thin films condensed on a Ag(110) substrate has been studied using a combination of reflectance absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption(TPD). At 100 K, styrene polymerizes upon ultraviolet irradiation. Postirradiation TPD indicates that several species form in the film. These species are identified as styrene dimer, trimer, and tetramer. RAIR spectra as a function of temperature provide evidence for the formation of polystyrene as well. A mechanism for the direct UV photopolymerization of styrene thin films is proposed.