Ambient-pressure X-ray photoelectron spectroscopy has been used to study the initial stages of dry thermal oxidation of Si₀.₆₀Ge₀.₄₀(001) grown epitaxially on Si(001). Chemical state resolved AP-XPS was performed at temperatures up to 300 ˚C and O₂ pressures up to 1 mbar. Comparisons were made to Si(001) and Ge(001). The Si 2p and Ge 3d core levels were monitored to study oxide growth in-situ. Experimental data was analyzed using the NIST Simulated Electron Spectra for Surface Analysis (SESSA) to determine oxide composition and thickness. Our analysis indicates that oxidation proceeds via three oxide growth rate regimes: rapid regime within the first 20 minutes of oxidation when the oxide was thin, a transitionary regime, and a quasi-saturated slow regime when the oxide was relatively thick. The oxidation of both Si and Ge during the rapid regime was found to be pressure dependent, where both rates decreased at lower O₂ pressures. The rapid regime oxidation rate for Ge was reduced compared to Si at lower O₂ pressures, resulting in significant suppression of GeO₂ formation compared to SiO₂. Results indicate that the rapid regime oxide growth rate at 300 ˚C was 96 and 65 Å/h for O₂ pressures of 1 and 0.01 mbar, respectively. This rapid regime rate was primarily driven by SiO2 - formation. At low pressure, significantly less Ge was incorporated into the oxide and the final oxidethickness was ~5 Å thinner. The Si and Ge oxidation rates in the slow regime were nearly the same for our experimental conditions. The slow regime growth rate at 300 ˚C was 3.3 and 3.9 Å/h for O₂ pressures of 1 and 0.01 mbar, respectively. Finally, the mixed oxide Ge fraction and thickness was found to be strongly dependent on rapid regime oxidation. An enhancement of the Si₀.₆₀Ge₀.₄₀(001) oxidation rate was observed relative to Si(001) and Ge(001), and this was for both the rapid and slow regimes.