The structure of the gallane adduct Me3P.GaH3 in the vapour and crystalline states has been investigated. The gas-phase electron-diffraction (GED) pattern has been analysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (r(h1) structure) were found to be: r(Ga-H) 159.0(11), (Ga-P) 244.3(6), r(P-C) 184.0(2), r(C-H) 108.3(7) pm; H-Ga-P 98.4(12) and Ga-P-C 117.7(3)degrees. The structure of a single crystal At 150 K shows that the adduct retains the same monomeric unit in the crystalline phase, with dimensions generally close to those of the gaseous molecule and an eclipsed conformation of the C3PGaH3 skeleton. The results are discussed and analysed in the light of quantum chemical calculations and of the properties of related adducts of Group 13 metal hydrides.