The restriction of a free-radical reaction to a single elementary step has been achieved by means of a rotating cryostat. Beams of vapour of halohydrocarbon and sodium are condensed upon opposite sides of the exterior surface of a rapidly rotating stainless-steel drum which contains liquid nitrogen. Each reactant deposits on a freshly formed surface of the other. The halohydrocarbon layer is about nine monolayers thick, but the sodium layer is so thin that it is discontmuous, there being only one atom of sodium to approximately twenty-five surface molecules of the halohydrocarbon. Radicals formed by the reaction RX + Na$\cdot \rightarrow$ R$\cdot$ + NaX are trapped on this surface, surrounded by unreactive neighbours. As deposition continues they are covered over, and a structure like a Swiss roll is built up in which radicals are sandwiched between layers of excess halohydrocarbon. By suitable choice of halohydrocarbon a specific radical may be prepared, and the method has proved to be a general one for the chemical preparation of hydrocarbon radicals. Electron spin resonance spectra of the following radicals have been recorded: propyl, butyl, heptyl, phenylethyl, s-butyl, isobutyl, t-butyl, hept-4-yl, cyclopentyl, cyclohexyl, allyl, phenyl and benzyl. The spectra of the alkyl radicals are discussed in relation to molecular configuration.