Dimethylamine-gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N.GaH3 molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (r(hl) structure) were found to be: r(Ga-N) 202.6(2), r(Ga-H) 155.6(8), r(N-C) 148.0(3), r(C-H) 111.2(6) pm; Ga-N-Ga 90.7(1), C-N-C 109.3(5), N-C-H 109.9(10) and H-Ga-H 119.4(42) degrees.