Mixtures of trimethyltin hydride and halogenoalkanes have been irradiated in the gas phase by a medium pressure mercury lamp. A very rapid reaction ensues in which a halogen atom in the original halogenoalkane is displaced by a hydrogen atom and trimethyltin halide is formed. The reaction is interpreted as a chain process involving halogen abstraction by trimethyltin radicals. Competitive expėriments have been carried out and in general the reaction is very unselective; the different halogens are abstracted from similar sites in the order Br > Cl > F and the reactivity of different sites lies in the order tertiary > secondary > primary. The relative rates of chlorine abstraction from a series of molecules with the formula RCCl3 has been determined. If R is electron attracting, chlorine abstraction is facilitated and if R is electron repelling chlorine abstraction is retarded. These results are considered to be evidence of a polar effect exactly opposite to that observed in reactions involving hydrogen abstraction by chlorine atoms.