The kinetics of the formation of films by the thermal decomposition of methylene chloride on an iron surface is investigated using a microbalance where the film thickness X is shown to vary with time t as X = X(m)(1 - exp(-Bt)) where X(m) is the maximum film thickness. X-ray photoelectron, Auger, and Raman spectroscopic analyses show that the film consists of an iron chloride and carbon, where Raman spectroscopy indicates that the carbon is present as small (similar to 50 A) particles. The observed kinetics are rationalized by assuming that growth is limited by thermal decomposition of methylene chloride at the gas-phase/film interface rather than transportation through the film and that the sites for thermal decomposition become poisoned as the reaction proceeds. Surface spectral analysis indicates that both the amount of surface carbon and the proportion of covalently bonded chlorine increase during film growth so that either the formation of a carbonaceous layer or the accumulation of an organochlorine species could be responsible for the cessation of growth.