The reactions of H and D atoms, produced by the discharge method, with benzene, four cycloalkanes, and the corresponding normal paraffins have been investigated at room temperature. Methane is the major product of each reaction. Collision yields have been calculated on the basis of hydrocarbon disappearance and exchange. Of the cycloalkanes, cyclopropane is the most inert with cyclopentane the most reactive. Cyclopropane and cyclobutane do not exchange with D atoms but benzene, cyclopentane, and cyclohexane undergo considerable exchange. In the case of the normal paraffins the degree of exchange increases with increasing molecular weight.It is concluded that the initial reactionis responsible for the products formed in the reactions but other primary processes may be responsible for the exchange of these hydrocarbons.