Nanoidentation responses of Al2O3 coatings obtained using chemical vapour deposition on cemented carbides were studied and related to the Al2O3 modification and microstructure. Five different Al2O3 layers were studied: (i) ?-Al2O3, (ii) a-Al2O3, obtained through the ? ? a phase transformation, (iii) as-grown a-Al2O3 with (101-2) texture, (iv) as-grown a-Al2O3 with (101-4) texture and (v) as-grown a-Al2O3 with (0001) texture. All the Al2O3 coatings were deposited to a thickness of at least 6 µm onto identical WC - 6 wt.% Co cemented carbide substrates. Nanoindentation was performed on taper-section specimens. The microstructure, phase and texture were elucidated by using transmission electron microscopy, scanning electron microscopy and X-ray diffraction. Nanoindentation responses of the as-grown, textured a-Al2O3 coatings were compared with ?-Al2O3 and a-Al2O3 formed as a result of the ? ? a phase transformation. The as-grown, textured a-Al2O3 layers exhibited clearly a higher hardness and Young's modulus than the ?-Al2O3 and the a-Al2O3 layers formed as a result of the phase transformation. Only slight differences in hardness and modulus could be detected between the (101-2), (101-4) and (0001) textured a-Al2O3 films. Among these the (101-2) textured a-Al2O3 coating exhibited the lowest hardness and modulus. Opposite to earlier reports, the present investigation clearly showed that ?-Al2O3 exhibited both the lowest hardness and Young's modulus as compared with the other studied coatings. It is emphasised that the earlier nanoindentation investigations that were intended to deal with a-Al2O3 were in fact performed on transformed ?-Al2O3. © 2007 Elsevier B.V. All rights reserved.