A good dielectric layer on the GaAs substrate is one of the critical issues to be solved for introducing GaAs as a candidate to replace Si in semiconductor processing. In literature, promising results have been shown for Al2O3 on GaAs substrates. Therefore, atomic layer deposition (ALD) of Al2O3 has been studied on GaAs substrates. We have been investigating the influence of the ALD process (thermal vs plasma-enhanced ALD) as well as the influence of the starting surface (no clean vs partial removal of the native oxide). Ellipsometry and total X-ray reflection fluorescence were applied to study the growth of the ALD layers. Angle-resolved X-ray photoelectron spectroscopy was used to determine the composition of the interlayer. Both processes were shown to be roughly independent of the starting surface with a minor dependence for the thermal ALD. Thermally deposited ALD layers exhibited better electrical characteristics based on capacitance measurements. This could be linked to the thinner interlayer observed for thermally deposited Al2O3. However, the Fermi level was not unpinned in all cases, suggesting that more work needs to be done for passivating the interface between GaAs and the high-k layer.