BaTi ₄ O ₉ with a unique pentagonal prism tunnel structure, has been reduced in H ₂ at temperatures between 1073 and 1173 K and then oxidised in O ₂ at 773 and 1173 K. The structural changes and the capability for radical formation on UV irradiation have been investigated. The intensity of an EPR signal (g = 2.018, 2.004), which appeared at 77 K upon UV irradiation in the presence of O ₂ , decreased on reduction at increasing temperatures, but was nearly restored by oxidation at 1173 K, after reduction at temperatures below 1123 K. In a photocatalytic system of RuO ₂ /BaTi ₄ O ₉ , reduction of BaTi ₄ O ₉ resulted in a remarkable decrease in photocatalytic activity for water decomposition, however, after oxidation at 1173 K the activity returned to the original level for reduction temperatures <1123 K. There was good correlation between the EPR signal intensity and the photocatalytic activity. Wavelength dependence of the EPR signal was in agreement with that for photocatalytic activity. The photocatalytic reaction is concluded to proceed at the same site as the radical formation. A model for an active site is proposed.