The 248 nm photochemistry of methyl iodide thin films was studied using reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and time-of-flight quadrupoIe mass spectrometry (TOF-QMS). The formation of predominantly CH2I2 and CH4 and some C2H6, CH3CH2I, CHI3, and I-2 photoproducts retained in the film was characterized by RAIRS and TPD. The integrated areas of the IR absorption bands for the two major photoproducts, CH2I2 and CH4, increase to a maximum and then decrease as photolysis of the film proceeds. A cross section for the loss of CH3I by 248 nm photolysis of the film was measured to be (1.6 +/- 0.1) x 10(-19) cm(2), approximately I order of magnitude lower than the gasphase cross section. At all laser fluences used in this study, CH3, I, and CH3I were ejected into the gas phase. The CH3 TOF distribution showed the signature of gas-phase CH3I photodissociation dynamics-two sharp peaks corresponding to the production of iodine atoms in the I(P-2(3/2)) and I*(P-2(1/2)) states. The TOF distributions of I and CH3I were fit by Maxwell-Boltzmann distributions corresponding to temperatures of 1400 and 1170 K, respectively. Three other species-CH4, I-2 and CH2I2-were observed in TOF-QMS, but only at higher laser fluences. It was determined that the I-2 and CH2I2 species are most likely fragments of a larger molecule, perhaps a cluster species, that photodesorbs as the film becomes enriched with photoproducts. The mechanism for CH4 photoejection appears to be of a different nature. The photochemistry of methyl iodide thin films can be understood in terms of a combination of photoprocesses occurring in the film and at the film surface.