The oxidation behavior of columnar and equiaxed Cr2AlC coatings in air at 990 °C is studied. During the transient heating-up period to 990 °C the two coating morphologies exhibit different oxidation behavior. In columnar coatings, faster Al diffusion along column boundaries, triple junctions, and intercolumnar pores leads to the formation of metastable alumina polymorphs, resulting in rapid oxidation. In contrast, equiaxed coatings exhibit slower oxidation kinetics due to limited Al mobility, leading to direct formation of passivating α-(Al,Cr)2O3 through oxidation of Cr and Al at the surface.