The "mixed" gallium-boron hydride HGa(BH4)(2) in its isotopically natural and perdeuteriated versions has been characterized both experimentally, by gas-phase electron diffraction and vibrational spectroscopy, and through ab initio [MP2(fc)/SV(d) level] and density functional theory [Becke3LYP/TZ(d, p) level] calculations. The vapor consists of monomeric HGa(BH4)(2) molecules with a single terminal Ga-H bond and two bidentate BH4 groups. The electron-diffraction pattern of the vapor has been remeasured using an all-glass inlet system. The theoretically favored model with C-2v symmetry provides the most satisfactory fit to the data. The following parameters (r(a) structure, distances in pm, angles in degrees; H-t terminal H atom, H-b bridging H atom) have been determined : r(Ga-H-t) 150(4), r(Ga-H-b) 178.8(7), r(Ga-B) 218.5(2), r(B-H-t) 118.4(4), r(B-H-b) 126.7(4), and B-Ga-B 119.2(15). The physical and spectroscopic properties of the compound imply that loose aggregation occurs in the condensed phases.