In this paper, we investigate the electrochemical formation of layers of self-organized high aspect ratio TiO2 nanotubes, grown by anodization of Ti in viscous electrolytes at different temperatures. In electrolytes consisting of glycerol or ethylene glycol with small additions of fluorides, the nanotube morphology can be strongly influenced by the viscosity, the electrolyte temperature and the applied potential. Compared with water-based electrolytes, in viscous electrolytes much longer and smoother tubes can be grown. Depending on experimental conditions, the nanotubes have an aspect ratio reaching up to about 150 for glycerol and 32 for ethylene glycol. The results further show that in glycerol electrolytes the current efficiency for nanotube formation is close to 100% (this is significantly higher than for comparable water-based electrolytes). (c) 2006 Elsevier Ltd. All rights reserved.