Photocatalytic nitrogen oxides (NOx) trapping is a new approach to reduce NOx emission from automobiles. The modification of TiO2 with Ba (Ba-TiO2) improves the NOx storage capacity (NSC) of TiO2 . X-ray absorption spectroscopy shows that Ba-TiO2 consists of isolated Ba species and bulky Ba(NO3)(2). Diffused reflectance infrared Fourier transform spectroscopy reveals that isolated Ba species affect the NOx trapping ability on neighboring Ti sites and enhance the photocatalytic NSC on Ba-TiO2.