The absorption spectra of TiO 2 and Au–TiO 2 have been measured as a function of calcination temperature in order to study the effect of Au on the band gap energy of TiO 2. Our results indicate that a signature of the presence of Au is a decrease in the band gap energy of TiO 2 to a value less than the commonly quoted band gap energies of bulk anatase and rutile. In addition, a drastic decrease in the band gap energy of Au–TiO 2 is observed with samples heated at 700 °C in an argon atmosphere, which is attributed to the influence of defects in the structure of TiO 2. The reduction in the band gap energy of TiO 2 in the presence of Au is correlated to enhanced photo chemical processes previously described and is associated with a strong interaction between Au and TiO 2.