The reaction of HP(SiMe3)2 with Me2AlH was studied by variable temperature NMR. Spectral data indicate that HSiMe3-elimination is the preferred mode of reactivity for producing the Me2AlP(H)SiMe3 moiety. However, this Al—P intermediate subsequently undergoes a transphosphination reaction with HP(SiMe3)2 to produce several Me2AlP(SiMe3)2-containing species. The major product from the reaction is the mixed trimeric species [Me2AlP(H)SiMe3]2•Me2AlP(SiMe3)2 (1), which was isolated and characterized. © 1995, American Chemical Society. All rights reserved.