Gallaborane (GaBH6, 1), synthesized by the metathesis of LiBH4 with [H2GaCl](n) at ca. 250 K, has been characterized by chemical analysis and by its IR and H-1 and B-11 NMR spectra. The IR spectrum of the vapor at low pressure implies the presence of only one species, viz. H2Ga(mu -H)(2)BH2, with a diborane-like structure conforming to C-2v symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements afforced by the results of ab initio molecular orbital calculations. Hence the principal distances (r(alpha) in Angstrom) and angles (angle (alpha) in deg) are as follows: r(Ga . . .B), 2.197(3); r(Ga-H-t), 1.555(6); r-(Ga-H-b), 1.800(6); r(B-H-t), 1.189(7); r(B-H-b), 1.286(7); angleH(b)-Ga-H-b, 71.6(4); and angleH(b)-B-H-b, 110.0(5) (t = terminal, b = bridging). Aggregation of the molecules occurs in the condensed phases. X-ray crystallographic studies of a single crystal at 110 K reveal a polymeric network with helical chains made up of alternating pseudotetrahedral GaH4 and BH4 units linked through single hydrogen bridges; the average Ga . . .B distance is now 2.473(7) Angstrom. The compound decomposes in the condensed phases at temperatures exceeding ca. 240 K with the formation of elemental Ga and H-2 and B2H6. The reactions with NH3, Me3N, and Me3P are also described.