The morphology and composition of anodic films, formed on aluminium at various current densities, in the range 1-$100{;}Am^{-2}$, in the molten bisulphate melt at different temperatures (418-498K), have been studied using transmission electron microscopy of ultramicrotomed film sections, and ion beam thinned films. From the structural analysis of the electron diffraction patterns taken from the ultramicrotomed sections and ion beam thinned films, it can be concluded that the crystal modification process from ${gamma}-Al_2O_3{;}to{;}{alpha}-Al_2O_3$ proceeds in the following steps : (equation omitted)