Isothermal annealing behaviour of intrinsic amorphous layers produced by stoichiometric implantation in a—axis oriented α—Al2O3 single crystals has been studied. The amorphous phase transforms directly to α—Al2O3 at a well defined planar interface which moves towards the free surface. The epitaxial growth slows down after initial rapid crystallization, indicating two separate regimes. The interface velocity shows Arrhenius behaviour in both regimes with activation energies of 0.6 and 0.08 eV respectively. There is an evidence for additional surface or random crystallization into κ or γ-Al2O3 phases within the first few nm on the surface, after prolonged annealing. These results are remarkably different from those reported previously for c–axis oriented Al2O3 crystals, showing the importance of substrate orientation during crystallization. A tentative model to explain the crystallization behaviour is discussed.