The surface reactions between trimethylaluminum (TMA) and deuterated water (D2O) in the deposition of Al2O3 were studied by using a mass spectrometer to determine the amount of the reaction product, methane (CH3D), produced in a flow-type atomic layer deposition (ALD) reactor during each deposition step. At low temperatures (<200 degrees C) more CH3D was produced during the TMA pulse whereas at;higher temperatures when the dehydroxylation is more extensive more CH3D was produced during the D2O pulse. Iri addition to the temperature, the hydroxyl coverage was also noticed to depend on the D2O dose, since D2O reacts with dehydroxylated alumina sites producing adsorbed -OD groups. According ed our results, which agree quite well with previous studies carried out by other methods, the dehydroxylation of the alumina surface seems to play a key role in the surface chemistry of the atomic layer deposition of Al2O3.