This talk deals with the relationship between the photocatalytic activity and the band structure of perovskite lanthanum titanium oxide (La₂Ti₂O₇). Nitrogen doping results in shift of the valence band, leading to band gap narrowing. This renders the visible-light responsive photocatalytic activity of lanthanum titanium oxide. Transient absorption spectroscopy has been used to investigate the charge dynamics in both the pristine and the N-doped perovskite lanthanum titanium oxide.