In situ x-ray photoelectron spectroscopy was performed during thermal desorption of a protective As layer and subsequent atomic layer deposition (ALD) of Al2O3 on In0.53Ga0.47As(001). H2O dosing on the As-decapped surface caused formation of As oxides and As hydroxides, which were reduced by a subsequent trimethylaluminum (TMA) pulse. However, when a TMA pulse was performed first, the In0.53Ga0.47As(001) surface did not oxidize during subsequent ALD cycles, suggesting passivation by TMA adsorption at water-reactive sites. Scanning tunneling microscopy performed on a structurally-similar InAs(001) surface after H2O dosing revealed that surface defects are created by displacement of surface As atoms during oxidation. These surface defects act as interface states, consistent with the inferior capacitance-voltage characteristics of H2O-first ALD-Al2O3 capacitors compared to TMA-first samples.