Nitrogen was doped into TiO2 by an ion implantation technique to investigate the depth distribution of nitrogen most effective for fabricating a visible-light responsive photocatalyst. In the nitrogen-doped TiO2 samples, the photocatalytic activity under visible-light irradiation was enhanced, wherein two types of chemical state of nitrogen, one photocatalytically active and the other inactive, were found depending on the nitrogen concentration, using electron energy loss spectroscopy. The depth distributions of nitrogen were quantified, so that the implanted nitrogen increased with thickness from the surface up to 90 nm, consistent with Monte Carlo simulations. The critical nitrogen concentration for the photocatalytically active species to form was estimated to be no higher than 1.8 atom%. We also derived semi-empirical equations from the present results to estimate the critical depth of nitrogen distribution effectively reactive to be 13 ± 5 nm from the surface. Copyright © 2014 John Wiley & Sons, Ltd.