The novel mixed hydride 2-galla-arachno-tetraborane(10), H₂GaB₃H₈, has been synthesised by metathesis between neat monochlorogallane and tetra-n-butylammonium octahydrotriborate, [Buⁿ₄N]⁺[B₃H₈]^–, at temperatures near 243 K. The compound has been characterised by chemical analysis, by its vibrational, ¹H and ¹¹B NMR, and mass spectra, and by its reaction with ammonia. Electron-diffraction measurements, carried out on the vapour at ca. 273 K, are wholly consistent with the tetraborane(10)-like structure of the molecule with gallium replacing boron at the 2 position. The r_a structure features the following parameters: r[Ga–B(1,3)] 231.2(0.1), r[B(1,3)–B(4)] 185.2(1.3), r(Ga–H_t) 144.2(1.1), r(Ga–H_b) 176.0(2.8) and r(B–H_b) 126.4(0.7) pm; and 114.4(0.6)° for the dihedral angle between the planes GaB(1)B(3) and B(1)B(3)B(4)(H_t= terminal H atom; H_b= bridging H atom). The NMR spectra of solutions imply non-fluxional behaviour of the molecule in the temperature range 193–283 K. Gallatetraborane(10) decomposes at ambient temperatures to give either metallic gallium, dihydrogen and tetraborane(10) or a solid with the composition GaB₂, dihydrogen and diborane according to whether it is in the condensed or gaseous phase, respectively.