The degassing behaviour and surface characterization of Al-Mg base alloys has been investigated using quadrupole mass spectrometry (QMS) and X-ray photoelectron spectroscopy (XPS). The alloy composition, particle size and the nature of the atomizing gas have been studied in terms of gas evolution and surface composition. XPS has been used both to measure oxide thicknesses and magnesium enrichment ratios. XPS results show that magnesium segregation increases for larger particle sizes and this is supported by QMS, with a correspondingly higher hydrogen evolution on heating being observed for the larger size fractions. High-resolution XPS of the carbon Is photoelectron peak (C1s) indicates the presence of carbonate component on the as-received magnesium-containing powders. This component is less pronounced on degassed powders indicating the evolution of CO2 on heating. This observation is supported by thermodynamic calculations.