Electrochemical reductive doping is considered to be a simple and effective method to improve the electric properties of TiO2 nanotube arrays (NTAs). We report on electrochemical tests of the reductive reaction of anatase TiO2 NTAs in aqueous and aprotic solvents. It is confirmed that hydrogen is not necessary for the electrochemical reduction of TiO2, and the reaction can be performed in either aqueous or aprotic solvents, where the former ismore effective. Three equivalent circuit models were also built to understand the electrochemical behavior, via electrochemical impedance, of pristine, reduced, and post-oxidized TiO2 NTAs in contact with electrolyte. Ti4+ is reduced to Ti3+ by electrochemical treatment to form self-doped TiO2 with metallic properties at the initial stages, and the reduced TiO2 NTAs can be easily re-oxidized to form a stable semiconductor when exposed in air. Two different reaction mechanisms performed in aqueous and aprotic solvents are discussed. These findings help to understand the complexity of the processes involved in the electrochemical reductive doping of TiO2 NTAs and suggest a way to improve the electronic properties of TiO2 nanomaterials. (C) 2017 The Electrochemical Society. All rights reserved.