In this work, the effect of the thermal annealing on the composition of crack-free anodic tin oxide layers with different channel diameters, formed in 1M NaOH at 2 and 4 V, was investigated in detail. STEM tomography confirmed that anodic oxides formed at 2 V have narrower channels, higher porosity, and higher surface area than those anodized at 4 V. Structural properties of oxides films were studied using different techniques such as X-ray diffraction, Raman spectroscopy, and UV-Vis diffuse reflectance spectroscopy. In addition, Raman imaging was employed to study local changes in the composition of anodic films. The photoelectrochemical performance of as-formed and annealed oxides was correlated with applied anodizing potential and annealing conditions. It was found that as-formed anodic oxides are composed of poorly crystalline SnO2-x with a significant amount of Sn2+ defects. When the oxide layers were annealed at 200 degrees C, crystallization of the SnO phase was observed, however, the amount of Sn2+ defects was not drastically reduced. A significant reduction of Sn2+ defects occurred at 400 degrees C due to a formation of intermediate Sn3O4 oxide and, in the final result, more stoichiometric and crystalline SnO2 was formed. We proved that appropriate design of anodizing and annealing procedures can lead to the formation of nanoporous tin oxide layers with controlled morphology, composition and, in consequence, photoelectrochemical properties. (C) 2019 Elsevier Ltd. All rights reserved.