In the present work we demonstrate that amorphous anodic TiO 2 nanotubes show upon annealing a tube diameter dependent phase stabilization. For tube diameters <30nm the formation of rutile is observed while for larger diameters anatase is formed. This effect is induced by the presence of the metallic substrate — in its absence all tube diameters are converted to anatase. As the nature of the stable phase strongly affects electronic, optical and biomedical properties of TiO 2 , the finding is of a high significance for many applications of TiO 2 nanotubes.