Anodic oxidation of Al-Zr bilayer samples (aluminium deposited on zirconium) has been investigated by secondary ion mass spectrometry depth profiling and transmission electron microscopy of ultramicrotomed sections. Anodizing proceeds in the sequence of growth of amorphous anodic alumina and, when the aluminium layer is consumed, growth of crystalline anodic zirconia on zirconium. Despite the outer anodic Al2O3 layer having a higher ionic resistivity than the inner anodic ZrO2 layer, fingers of ZrO2 do not penetrate locally into the outer Al2O3 layer. The oxidation of the inner zirconium layer results from oxygen ions transported through the outer amorphous Al2O3 layer without zirconium being in direct contact with the electrolyte. These new findings and their impact on the detailed understanding of oxygen transport during the growth of crystalline anodic ZrO2 are considered.