Methyl bromoacetate and ethyl 2-bromopropanoate are reduced by Bun3P→BH3 or Bun3P→BH2Ph to methyl acetate and ethyl proponoate, respectively, in chlorobenzene at 80–110 °C in the presence of dibenzoyl peroxide or t-butyl perbenzoate. Amine complexes of borane or phenylborane are much less effective reducing agents. The reductions may also be initiated photochemically and are inhibited by a phenolic radical scavenger. A homolytic chain mechanism is proposed in which the phosphine–boryl radical abstracts halogen from the bromo ester and is subsequently regenerated by reaction of an α-(alkoxycarbonyl) alkyl radical with the phosphine–borane. The latter propagation step, together with halogen abstraction from Rl and addition of the derived alkyl radical to the CC bond, is also involved in the chain reaction between Bun3P→BH2Ph, an alkyl iodide, and ethyl acrylate according to equation (A); Bun3P→BH3 reacts similarly but gives lower yields of ester. Reaction (A) proceeds smoothly at 110 °C Bun3P→BH2Ph + Rl + CH2CHCO2Et→RCH2CH2CO2Et + Bun3P→BHIPh (A) when initiated by t-butyl perbenzoate and moderate yields of isolated esters were obtained from n-butyl iodide, cyclohexyl iodide, and 3β-iodocholest-5-ene. This last iodide gives an epimeric mixture of 3α-and 3β-esters in total isolated yield of ca. 50%. Similar addition reactions take place between Bun3P→BH2Ph, Bunl, and diethyl vinylphosphonate or phenyl vinyl sulphone. It is concluded that Bun3P→BH3 and particularly Bun3P→BH2Ph offer promise as alternatives to tin, mercury, and germanium hydrides in radical chain reactions of synthetic value.