In this work, two different CVD Al 2 O 3 coatings have been investigated: (i) as-deposited α-Al 2 O 3 and (ii) α-Al 2 O 3 produced by heat treatment of as-deposited -Al 2 O 3 . The aim was to describe and compare the detailed microstructure of these two α-Al 2 O 3 coatings using transmission electron microscopy (TEM). In the as-deposited α-Al 2 O 3 layer, the grains were columnar with a width of 1-2 μm, and a length of up to 5 μm. The grains in the transformed α-Al 2 O 3 layer were equiaxed, with a grain size of about 1 μm. Both α-Al 2 O 3 layers contained a large number of dislocations and voids. Linkage of voids was most apparent in the transformed α-Al 2 O 3 layer, which also contained a number of large cracks. During the -Al 2 O 3 -> α-Al 2 O 3 phase transformation, the columnar grains in the -Al 2 O 3 layer were transformed into columns of smaller equiaxed α-Al 2 O 3 grains, separated by low angle grain boundaries. It was concluded that the close-packed oxygen ion planes are preserved during the -> α phase transformation, but the stacking sequence of these planes is changed from ....ABAC.... as in -Al 2 O 3 to ....ABAB.... as in α-Al 2 O 3 .