A series of titania (TiO2)-based nanometer-sized photocatalysts, including nitrogen-doped TiO2 nanoparticles (NPs), are investigated using X-ray photoelectron spectroscopy (XPS). Conclusive evidence is obtained for O-Ti-N bond formation during the doping process. Therefore, this substitutional doping is held accountable for the significant increase in photocatalytic activity in nitrogen-doped TiO2 NPs.