The field of alkyl radical reactions has been reviewed. Most of the work, including results presented in this thesis, has been carried out in the gas phase, but where liquid phase studies have been made, they are included and compared with the results from the gas phase. Emphasis has been placed on quantitative experiments, and tables have been drawn up summarising the most reliable values of rate constants, rate constant ratios, Arrhenius parameters, etc., for the following classes of alkyl radical reaction: combination of like and unlike radicals; disproportionation; metathesis; addition to multiple bonds; and decomposition. Alkyl radical isomerisation has also been considered. The photoinitiated chain decomposition of aldehydes has been extended and developed as a general method of studying the reactions of alkyl radicals. This has been possible by gas chromatographic analyses of the products. Quantitative information has been obtained by photolysing propionaldehyde, n- butyraldehyde, iso- butyraldehyde and n- valeraldehyde for ethyl, n- propyl, s- propyl and n -butyl radicals respectively, in reactions of all the above classes. By photolysing mixtures of aldehydes, the cross- combinations of ethyl with propyl and propyl with n -butyl radicals have been studied, and the results obtained, taken in conjunction with information in the literature, have led to general conclusions on alkyl radical combinations. The cross-disproportionation reactions of methyl with s- propyl and t -butyl radicals have been investigated by photolysing mixtures of acetone with methyl isopropyl ketone and pinacolone. The results of these studies considered with analogous information in the literature have been treated to yield infinformation on the nature of the activated complex in a disproportionation reaction.