The relative stability of the radicals that can be produced from amine–boranes and phosphine–boranes is investigated at the G3-RAD level of theory. Aminyl ([RNH].:BH3) and phosphinyl ([RPH].:BH3) radicals are systematically more stable than the boryl analogues, [RNH2]:BH2. and [RPH2]:BH2.. Despite similar stability trends for [RNH].:BH3 and [RPH].:BH3 radicals with respect to boryl radicals, there are significant dissimilarities between amine– and phosphine–boranes. The homolytic bond dissociation energy of the NH bond decreases upon association of the amines with BH3, whereas that of the PH bond for phosphines increases. The stabilization of the free amine is much smaller than that of the corresponding aminyl radical, whereas for phosphines this is the other way around. The homolytic bond dissociation energy of the BH bond of borane decreases upon complexation with both amines and phosphines.