The oxidation of Re/ γ-Al 2O 3 catalysts, containing 1 and 10 wt% of rhenium, sintered in hydrogen was examined in the temperature range of 20–800 °C. The structures of the catalysts were investigated by XPS spectroscopy, TEM, and O 2 uptake measurements. The low-loaded catalyst comprises metallic particles with sizes of 1–4 nm ( d av=2.1 nm), while the high-loaded catalyst comprises particles with sizes of 1–9 nm ( d av=4.9 nm). Even short exposure to air at room temperature causes complete oxidation of small clusters of metallic Re, while larger particles are covered with very thin ReO x skin (undetected by TEM). XPS shows that the high-loaded catalyst still contains 94.5% of metallic Re, while the low-loaded catalyst contains only 60.5%. The remaining part of the Re is oxidized to Re 4+, Re 6+, and Re 7+ species. Oxidation at 150 °C causes enhanced formation of Re 4+–Re 7+ species and the amount of metallic Re quickly decreases to 33 and 2% for high- and low-loaded catalysts, respectively. This indicates a high affinity of the highly dispersed Re to oxygen. At this temperature, the Re/Al atomic ratio increases 2–4 times, indicating a large spreading of the oxide species on the support surface. Simultaneously, the average size of Re particles decreased as determined by TEM. At 300 °C, whole Re was oxidized mainly to Re 2O 7, though some amount to Re 4+ and Re 6+ species remained. The O 2 uptake measurements confirm oxidation of rhenium particles. For the high-loaded catalyst O 2 uptake attained a maximum level (O/Re=3.3) already at 300 °C, while for the low-loaded catalyst even at 500 °C the uptake (O/Re=2.98) is below the maximum level. XPS data showed, however, that at 500 °C, oxidation of rhenium to Re 2O 7 occurs for both catalysts. The Re/Al atomic ratio remains nearly constant after oxidation of both catalysts at 300–800 °C, indicating that Re 7+ species are firmly bonded to alumina surfaces even at 800 °C. A detailed mechanism of oxidation of Re particles with different sizes is proposed based on a quantitative analysis of the XPS, O 2 uptake, TEM, and previous Raman results.