Direct reactions between BX3 (X = Me, F, Cl) and the methylhydrazines MeNHNH2, Me2NNH2, Me2NNHMe, and Me2NNMe2 yield 1:1 adducts in ail cases except (a) BMe3 and Me2NNMe2, which do not react; (b) BCl3 and MeNHNH2, Me2NNH2 or Me2NNHMe, which result in hydrazinolysis of the B—Cl bond; and (c) BCl3 with Me2NNMe2, which yields a dark brown, viscous liquid. However, the unstable 1:1 adducts Me2NNH2BCl3 and Me2NNHMeBCl3 were prepared in solution, but were not isolated. On the basis of n.m.r. studies, it is concluded that adduct formation occurs at the more highly methylated nitrogen atom of Me2NNH2 and Me2NNHMe; for MeNHNH2, two isomeric adducts are formed with BF3, in a 5:1 molar ratio, corresponding to complexation at the methylated nitrogen, and at the unsubstituted nitrogen atoms, respectively. A similar study of adduct formation between BX3 and the trifluoromethylphosphinohydrazines (CF3)2PNMeNMe2, (CF3)2PNHNMe2, (CF3)2PNMeNH2, and (CF3)2PNHNHMe indicates that 1:1 adducts are formed in all cases except (d) BMe3 with (CF3)2-PNMeNMe2, for which no reaction was observed; and (e) BCl3 with (CF3)2PNMeNH2 and (CF3)2-PNHNHMe, for which elimination of (CF3)2PCl was observed. The adducts are volatile white solids, except (CF3)2PNMeNH2BMe3, which is a colorless liquid at room temperature, and (CF3)2PNMeNMe2BCl3, which is a non-volatile solid. In only one case was it possible to deduce an unambiguous stereo-chemistry for the adduct from n.m.r. data, viz. (CF3)2PNMeNH2BX3, (X = Me, F) where complexation occurs at the terminal, least substituted nitrogen atom. In (CF3)2PNMeNMe2BF3 and in (CF3)2-PNHNHMeBF3, the NMe and the NMe2 groups, and the PNH and NNH protons, respectively, are deshielded by the same, or very similar extents, so that an exclusive choice of structure cannot be made.