The reactivity of primary phosphine-boranes RPH 2 BH 3 (R = Ph and Me) towards CH 2 =CHCO 2 Me and CH 2 =CHP(O)(OMe) 2 is discussed. Hydrophosphination is the major process. The presence of the free phosphine (0-20%) in the crude media indicates that a competetive hydroboration reaction also occurs. The P-H addition was found to be controllable to give, in reasonable yields, either the mono or the bis-adducts. All the adducts are stable and are fully characterized. The preparation of an unsymmetrical bis-adduct is also presented.