Corundum-structured aluminium oxide (α-Al2O3) coatings are widely used as protective and diffusion barrier coatings due to their outstanding properties, such as high hardness, good wear resistance, and unique high temperature performance even in hazardous environments. Even though α-Al2O3 films can properly be deposited via chemical vapour deposition, the required temperatures (800-900 °C) exclude the majority of important substrate materials. Therefore, it is of great interest to synthesise aluminium-based oxides in the corundum structure via physical vapour deposition (PVD). Nevertheless, when lowering the process temperature to ~550 °C, additional metastable oxide phases are formed. A successful approach to overcome this issue is the addition of Cr to the coatings, since Cr2O3 is corundum-structured with mutual solubility in α-Al2O3. Thus, Cr promotes the formation of corundum-type (Al,Cr)2O3 solid solutions. However, crystalline single-phased α-Al2O3 coatings still require Cr contents of at least 30 at.% (of the metals) when prepared by PVD.The higher the Al content the higher is the thermal stability, thus, research drives the development towards maximum Al content combined with corundum-type structure. Recently, we showed that the addition of up to 5 at.% Fe to industrially-relevant (Al0.70Cr0.30)2O3-based coatings leads to a significant increase of the corundum phase fraction at the expanse of metastable oxides. Therefore, we study in detail the oxidation mechanisms of (inter-)metallic Al-Cr-Fe and substoichiometric (Al,Cr,Fe)xOy oxide coatings synthesised by arc evaporation of either Al0.70Cr0.30, Al0.675Cr0.275Fe0.05, or Al0.63Cr0.27Fe0.10 targets. Based on our detailed SEM, XRD, and TEM studies we can conclude that the addition of Fe to the targets and thereby to the coatings has a marginal impact on the as-deposited phase formation and morphology of the γ2-Al8Cr5-type based intermetallic or substoichiometric oxide coatings. However, the addition of Fe promotes the formation of the corundum-type structure if the coatings are annealed in ambient atmosphere between 900°C to 1300°C.