A comprehensive study of gold ion-doped TiO2 and gold-deposited TiO2 was carried out by measuring the photo-activity, X-ray diffraction (XRD) spectra, photoluminescence (PL) spectra, and X-ray photo-electron spectra (XPS) of gold, oxygen and titanium. The results showed that anatase phase was the main component and that the electronic structure and interfacial electron transfer were significantly influenced by the presence of either gold ion or gold. The photo-activity was quantified in terms of methylene blue (MB) photo-degradation. As a result, the presence of gold ion in TiO2 lattices or gold on TiO2 surface greatly enhanced their photo-activity. The optimum molar content of gold ion doping and gold deposition was 0.5%. XPS measurements showed that there were Au, Au+, or Au3+ species in/on gold modified TiO2, and that Ti3+ species formed in TiO2 lattice owing to the presence of gold or gold ion. PL measurements showed that the recombination of electrons/holes was greatly hindered owing to gold or gold ion. The lower recombination of electron-hole pairs which had a good agreement with the greater photo-activity might be attributable to the presence of defect energy level, gold impurity energy level. or the presence of gold or gold ion. The photo-activity of gold modified TiO2 appears to be a complex function of the dosage of gold or gold ion, the distribution and electronic structure of gold or gold ion, and the defect or impurity energy level.