It is well known that the surface of metal oxide catalysts presents, usually, non-crystalline species containing impurities and low coordinated cations, which are observed in HRTEM images as a disordered layer of amorphous phase. Despite of being these species more accessible and less stable than the crystalline catalysts components, they have been rarely considered when analysing the catalyst activity. In this work, we have studied the effect of a treatment with HCl solution at pH = 0 on the characteristics of the disordered layer in two commercial anatase TiO2 samples, using techniques such as TGA, 1H MAS-NMR and HRTEM, and by determining these sample photocatalytic activity for the 4-nitrophenol degradation in aqueous suspension. The results indicate that associations of chlorine ions in amorphous titania chains and hydrated excess protons structures interact with anatase bridging hydroxyls. This interaction, that breaks Ti-O-Ti bonds between the anatase particles and the disordered layer species, enhances the anatase hydroxyls acidity, favouring the formation of 0-radicals and eventually increasing the photocatalytic activity for 4-nitrophenol degradation under UV irradiation. (C) 2017 Elsevier B.V. All rights reserved.