Hybrid density functional theory has been used to investigate the initial surface reaction mechanism in atomic layer deposition (ALD) of Al2O3 on the hydroxylated GaAs(001)-4 x 2 surface. The precursors for ALD of Al2O3' are trimethylaluminium (TMA) and H2O as the aluminium and oxygen sources. For the first half-reaction between TMA with the GaAs surface the calculated activation barrier is 15.4 kcal mol(-1), and the H2O half-reaction proceeds by a mechanism similar to that of the first half-reaction, resulting in an activation barrier of 27.6 kcal mol(-1). Both half-reactions are thermodynamically favourable, exothermic by 52.0 and 43.6 kcal mol(-1) relative to the reactants, respectively.