Abstract Methyl and i-propyl radicals have been found to react with methyl diphenylphosphinite to give respectively, methyldiphenylphosphine oxide and i-propyldiphenylphosphine oxide. t-Butyl radicals under similar conditions, initiated the transformation of the ester to methyldiphenylphosphine oxide. Benzyl radicals were found to be unreactive. Phenyl radicals, obtained from phenylazotriphenylmethane, were found to react with a number of alkyl diphenylphosphinites to give triphenylphosphinine oxide plus alkyltriphenylmethane. Cyclopentyl and cyclohexyl diphenylphosphinites were found to react abnormally in that they gave cycloalkene instead of cycloalkyltriphenylmethane. Reactions are postulated as occurring by attack of the alkyl radical upon the trivalent phosphorus ester to give a phosphoranyl radical which subsequently decomposes to give phosphine oxide plus an alkyl radical.