The structures and morphologies of Al-rich Al-Au alloy nanoparticles thermally oxidized under gaseous oxygen mixtures were investigated by transmission electron microscopy while varying the temperature and oxygen exposure conditions. The exposure of alloy nanoparticles to oxygen at room temperature formed amorphous alumina coatings that protected unoxidized core metals against further oxidation up to approximately 1000 A degrees C. Above this temperature, Au-gamma-Al2O3 Janus nanoparticles were generated as a result of complete oxidation. In contrast, the direct oxidation of preheated alloy nanoparticles without oxide coatings enabled complete oxidation to proceed at temperatures as low as 575 A degrees C. Under lower oxygen pressures, direct oxidation resulted in the formation of Au-gamma-Al2O3 Janus NPs with worm-like morphologies, suggesting anisotropic oxide growth via interfacial oxidation. In a similar manner, Pt-gamma-Al2O3 Janus NPs were generated by the direct oxidation of Al-Pt alloy NPs.