The foaming behavior of aluminum in air and argon was compared by means of in situ X-ray radioscopy. A faster collapse was observed in argon, suggesting a greater loss of the blowing gas through out-diffusion caused by a thinner oxide layer at the surface of the samples compared with the samples foamed in air. This interpretation is supported further by calculations that show a correlation between the gas loss and the thickness of the oxide layer. © The Minerals, Metals & Materials Society and ASM International 2010.