A mechanism is proposed to describe the initiation of optimized arrays of nanopores and nanotubes formed via anodization of The mechanism may be applicable to other valve metals in addition to titanium, Initiation is triggered by compositional changes in the oxide and electrolyte, near the interface region. that break down the planar surface. In the electrolyte, the compositional changes are enhanced by species (such as fluoride) that form complexes with metal cations. The planar instability is driven by the variation of the oxide's composition with distance from the oxide/electrolytic interface. Specifically, breakdown may occur in oxides with metal cation (M+z) transport coefficients that are significantly less than 1. resulting in M+z depiction at and near the oxide's surface. The proposed mechanism indicates that, in addition to the compositional gradient the initiation of nanopores is controlled by the potential gradient in the oxide as well as the oxide's dissolution rate. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.3049341] All rights reserved.