We present X-ray photoelectron spectroscopy (XPS) measurements of several Al-Cr-Fe samples which are mixtures of approximants of the decagonal phase. Some samples also contain a hexagonal gamma -brass phase. Our purpose is to evaluate the effect of chemical composition, particularly Cr content, on the response of the surface to oxidation. Under mild conditions only aluminium oxidizes, but under extreme conditions (water immersion at room temperature, or oxygen exposure at high temperatures), chromium oxidizes as well. XPS data also provide a measure of the oxide thickness. Cr has no discernible effect on oxide thickness when the oxidizing environment is the gas phase, but provides significant protection against water immersion, where high concentrations of Cr reduce the thickness by as much as 40%. These results for the Al-Cr-Fe samples are compared with results for approximants and quasicrystals in other systems.