Ordered anodic aluminum oxide (AAO) templates with pores of 12 nm in diameter and aspect-ratios (length-to-diameter) of >1000 are prepared by a simple two-step anodization process under mild-anodization conditions. Such a low diameter pores are consequence of the reduction of the dielectric constant of the electrolyte together with the effect of increasing the electrolyte viscosity. We show that the temperature has a strong impact on reducing the pore diameter, as provokes the increase of viscosity of the medium and thus the hindering of the diffusion of species involved in the widening of pores. Moreover, we manage to stop the AAO growth process at the pore nucleation stage and we show that 8 nm in diameter pores are originally formed under these conditions. The presented ultra-narrow pore templates may allow for the preparation of ordered arrays of high aspect-ratio ultra-small one dimensional (1D) nanostructures that could be useful to explore quantum phenomena and integrate quantum-devices. © 2013 Elsevier Inc. All rights reserved. ; The authors would like to acknowledge financial support from ERC 2008 Starting Grant “Nano-TEC” number 240497. C.V.M. acknowledges CSIC and European Union Funding for JAE PhD fellowship. ; Peer Reviewed