The phosphine oxide complexes [GaX3(Me3PO)] and [(GaX3)2{μ-o-C6H4(CH2P(O)Ph2)2}] have been prepared and characterised by microanalysis, IR and multinuclear NMR (1H, 13C{1H}, 31P{1H} and 71Ga) spectroscopy. The structures of [GaCl3(Me3PO)], [(GaBr3)2{μ-o-C6H4(CH2P(O)Ph2)2}] and of the ionic product [GaI2(Me3PO)2][GaI4] have been determined and show that the Lewis acidity of the gallium halides towards phosphinoyl ligands diminishes as the halogen becomes heavier. The [GaX3(Ph3E)] (X = Cl, Br or I; E = P or As) and [(GaX3)2{μ-o-C6H4(CH2PPh2)2}] (X = Br or I) have been prepared and their structural and spectroscopic properties compared with those of the phosphinoyl complexes. The results, and competitive solution NMR studies, show that Ga(III) binds the hard R3PO in preference to the softer phosphine or arsine ligands. Hydrolysis of gallium(III) phosphines is shown to lead to [R3PH][GaX4], but in contrast to some other p-block halides, GaX3 do not promote air-oxidation of R3P to R3PO.