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)°.