γ′-strengthened Co-base superalloys, based on the Co-Al-W system and its modifications, show high potential for use at high temperatures, with promising mechanical properties. A challenge remaining is the high-temperature oxidation behavior, as oxidation resistance above 800 °C is relatively poor as compared for instance to corresponding Ni-based superalloys. This is due to hindered formation of continuous, protective Al₂O₃ oxide layer. The presentation will discuss elementary mechanisms in the early stages of oxidation of Co-Al-W model alloys. A special focus of our studies is to clarify the role of the γ/γ'-microstructure in the formation and growth of oxide scales. In situ oxidation experiments in SEM, ex situ high-resolution TEM, and further surface analytical techniques were employed to determine critical factors and mechanisms of high-temperature oxidation of this alloy system. The results demonstrate the key role of the ratio of elemental fluxes (diffusion of oxygen and aluminum) on the formation of protective alumina scales.