A series of Bi2O3-TiO2 composite catalysts with variable quantities of bismuth was prepared by a single pot microemulsion procedure, calcined at 450 degrees C, and evaluated in the gas-phase degradation of toluene. Samples improve the reaction rate and quantum efficiency of the TiO2 anatase reference material by a maximum factor ca. 2.2 and increase significantly the selectivity to the total oxidation product, CO2, upon both UV and sunlight-type excitation. A complete bulk and surface structural and electronic characterization using X-ray diffraction (XRD), X-ray photoelectron (XPS), Raman, and UV-vis spectroscopies was carried out to interpret the catalytic results. The presence of bismuth enhances the optical response of the composite material to the visible region and modifies the morphology of the titania component. The study interprets the enhanced photoactivity of the composite materials with respect to both Ti anatase and the beta Bi2O3 single oxide reference systems as a compromise between two effects: the contact of anatase with a Bi oxide phase as well as the anatase morphology modification along the Bi-Ti series. Both effects influence the photo-chemical activity of the composite materials and provide the basis to explain the optimum performance achieved with a material containing a 5 wt.% of bismuth oxide.