Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al₂O₃ counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This talk will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H₂O. These films were then studied in Type 1 DI water, 2M NaCl, .5M NaCl, and .1M NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. We will discuss the possible effects of CO₂ dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Finally, we will show how a post-ALD plasma treatment to the film’s surface alters the hydration and dissolution behavior, improving stability in several aqueous conditions.