The aluminiumAluminium casthouseCasthouse is responsible for substantial amounts of CO2 emissionsEmissions. Substituting low-emissionsEmissions hydrogenHydrogen for fossil fuels such as LNG and LPG would significantly reduce the climate impact of aluminiumAluminium castingCasting. Such a fuel-switch would also change the combustion products, eliminating CO2 and increasing the amount of H2O, potentially up to 100% in the case of oxy-fuel combustion. As a critical factor in aluminiumAluminium castingCasting is the loss of metalMetals due to oxidationOxidation and dross formation, it will be necessary to have a good understanding of how the oxidationOxidation behaviour is influenced by the composition of the atmosphere. In this paper, we study how oxidationOxidation and dross formation on an AlSi7MgSr alloy changes for different atmospheres containing 40% H2O versus 20% O2. We find no temperature dependency of oxide and dross formation in the temperature range 670–750 °C. While inert process gas provides sufficient stirring for noticeable oxidationOxidation due to interactions with the laboratory air at the surfaceSurface, the use of oxidising process gas significantly increases dross formation, by a factor of 3–4. Comparing dross formation with 40% H2O versus 20% O2, we find that 20% O2 gives significantly more dross, despite the total molar amount of oxygen atoms being constant in these gas mixtures.