In casthouses, water vapor has long been considered a source of high hydrogen contents in aluminum melts. Moisture levels are usually highest in melting and holding furnaces, where H2O contents of up to 18 vol% occur due to the combustion of natural gas with air. In the case of oxyfuel burners, contents of even more than 60 vol% are theoretically possible. To avoid defects such as annealing bubbles or grain porosity in the subsequent production steps, the absorbed hydrogen must be reduced to a minimum by degassing treatments. To gain a better understanding of the interaction between liquid aluminum and water vapor, the hydrogen absorption of aluminum-magnesium melts under atmospheres with varying water vapor contents of 0–90 vol% at around 700 °C (1300 °F) is investigated in this work. Furthermore, the influence of H2O on the oxidation behavior is also examined.