Self-organized TiO2 nanotube arrays are characterized by gaps that separate the layers of oxide surrounding cylindrical pores. Nanotube layer growth is thought to involve oxide flow induced by mechanical stress associated with anodization. The mechanism of TiO2 nanotube initiation was studied with in situ stress measurements during anodization in ethylene glycol-water-fluoride solutions. Compressive stress changes due to initial growth of barrier oxide films were detected, along with significantly larger tensile and compressive stress changes that followed the appearance of gaps between nanotubes at the metal-oxide interface. Interrupted anodizing experiments revealed that the latter stress changes are due to chemical processes or open-circuit electrochemical reactions. Further evidence for such reactions was deduced from the calculation of apparent Ti valences below four from measurements of Ti metal consumption during anodization. The gaps between nanotubes are thought to result from local oxide delamination at the metal interface induced by oxide flow, and the large compressive stress change is attributed to chemical oxidation of Ti+3 ions in the nanotube walls. This previously unrecognized chemical oxidation reaction may suggests new avenues to control nanotube layer growth and properties. (C) 2018 Elsevier Ltd. All rights reserved.