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In this work, we have quantitatively elucidated the source of the hydrogen content in the atomic layer deposition of Al2O3 at different temperatures (80-220 °C), by replacing the H2O precursor with heavy water (D2O) to use as a tracer and discern between the H coming from the unreacted metal precursor ligands and that from the unreacted -OD (hydroxyl) groups coming from the (heavy) water. The main source of impurities arises from the unreacted hydroxyl groups (-OD), reaching ~18 atom % of deuterium at a deposition temperature of 80 °C. Reconsidering carefully our own and literature experimental data, we concluded that the generally accepted mechanism of steric hindering by monodentate Al(CH3)2 adsorbates (dimethylaluminum) cannot be solely responsible for the retention of hydroxyls during atomic layer deposition (ALD). On this regard, we propose two additional mechanisms that can lead to sterically hinder hydroxyl groups which will then remain unreacted in the film: surface rehydroxylation resulting in the reconfiguration of bidentate or tridentate adsorbates into monodentate adsorbates and hindered subsurface hydroxyl groups during the (heavy) water pulse and the hydroxylation of sterically hindered dissociated methyl chemisorbed species. Based on these three steric hindrance mechanisms, we constructed a growth model that consists of the initial chemisorption configurations of trimethyl-aluminum molecules with the alumina surface and the subsequent reconfiguration of the resulting adsorbates into a monodentate configuration that consequently leads to sterically hindered hydroxyl groups. The fraction of AlOx adsorbates arranged in monodentate and bidentate configurations entails a specific number of O/Al atoms and unreacted hydroxyl groups inside the film. This model was able to explain the deuterium content, the O/Al ratio, and the density obtained from Rutherford back-scattering and heavy ion elastic recoil detection analysis measurements. Furthermore, this model was able to predict more accurately the growth per cycle to what has been reported to be the ALD window of alumina. Our findings will spur further detailed investigations of the reaction and growth modes in ALD films. [ABSTRACT FROM AUTHOR]