The doping of Au/TiO₂ with FeO_x is shown to result in a strong enhancement of its photocatalytic activity in the degradation of the azo dye Orange II. In order to examine the source of this enhancement, Au-FeO_x/TiO₂ nanocomposites containing different molar ratios of Au:Fe were synthesized, and X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM) analyses indicated that the TiO₂-supported Au nanoparticles were partially covered with an amorphous layer of FeO_x species, in which the iron was present as Fe²⁺ and Fe³⁺. The metal-semiconductor system, i.e., Au/TiO₂, showed only a moderate degradation rate, whereas doping with FeO_x strongly enhanced the degradation activity. The bandgap energy decreased gradually from Au/TiO₂ (3.13 eV) to the catalyst with the highest FeO_x loading Au-FeO_x (1:2)/TiO₂ (2.23 eV), and this decrease was accompanied by a steady increase in the degradation activity of the catalysts. XPS analyses revealed that compared to Au/TiO₂, on Au-FeO_x/TiO₂ a much higher population density of chemisorbed and/or dissociated oxygen species was generated, which together with the decreased bandgap resulted in the highest photocatalytic activity observed with Au-FeO_x (1:2)/TiO₂. The processes occurring during reaction on the catalyst surface and in the bulk liquid phase were investigated using operando attenuated total reflection IR spectroscopy (ATR-IR) combined with modulation excitation spectroscopy (MES), which showed that the doping of Au/TiO₂ with FeO_x weakens the interaction of the dye with the catalyst surface and strongly enhances the cleavage of the azo bond.