The band alignment at the interface between GaAs and amorphous Al2O3 is studied through the use of hybrid functionals. For the oxide component, a disordered model is generated through density-functional molecular dynamics. The achieved structure shows good agreement with the experimental characterization. The potential line-up across the interface is obtained for two atomistic GaAs/Al2O3 interface models, which differ by the GaAs substrate termination. The calculated valence band offset amounts to 3.9 eV for an interface characterized by the occurrence of Ga-O bonds as dominant chemical bonding, favoring the high-energy side in the range of experimental values (2.6-3.8 eV). The effect of As antisite and As-As dimer defects on the band alignment is shown to be negligible. (C) 2015 AIP Publishing LLC.