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 th...