The reactions of R2PNMe2, Me2PNR/2, and (Me2N)nPMe3_nwhere R = Me, Et, Ph, and Cl, R' = Me, Et, Prn, Pr1, and SiMe3, and n = 1–3, with varying mole ratios of BH3.THF have been carried out and studied by using multinuclear NMR spectroscopy. Although P-B-bonded monoadducts Were always obtained, B-P-N-B-bonded bisadducts were also obtained for Me2PNMe2, Me2PNEt2, and Et2PNMe2. These are the first reported examples where the nitrogen atom in acyclic aminophosphines demonstrates reactivity toward BH3. The extent of bisadduct formation decreases dramatically in going from Me2PNMe2 to Me2PNEt2. Keq, ΔH, and ΔS values were obtained for the Me2PNMe2.BH3/H3BP(NMe2.BH3)Me2and Et2PNMe2.BH3/H3BP(NMe2.BH3)Et2equilibrium systems. The results are compared with those reported previously for analogous aminoarsines. A competition study involving the Me3N, Me3P, Me3As, Me2PNMe2, Me2AsNMe2, and BH3.THF systems is discussed relative to the nature of P-N and As-N bonding. © 1989, American Chemical Society. All rights reserved.