Novel bismuth molybdenum titanium oxide catalysts, prepared via different sol-gel routes, were studied by X-ray photoelectron spectroscopy (XPS) at ambient temperature after evacuation and after reaction with hydrogen or oxygen. The surface Bi3+ and Mo6+ species of several sol-gel catalysts could be reduced and reoxidized at room temperature, whereas the two reference materials gamma-bismuth molybdate and Bi2O3 were not reduced in hydrogen (10(-4) mbar) at temperatures below 573 K. The reduction of Bi3+ to Bi-0 was observed to be remarkably more facile than that of Mo6+ to Mo5+. Reduction of surface bismuth species in hydrogen and re-oxidation in oxygen could be followed by XPS at room temperature in the pressure range of 10(-6)-10(-4) mbar over a time scale of a few minutes to several hours. The facile reduction of surface Bi3+ is probably not connected to the bulk or surface composition. It is more likely due to the unique morphology of the bismuth molybdenum oxides stabilized by titania in the sol-gel materials. It emerged from XRD and Raman investigations that the mean crystallite size of Bi- and Mo-containing phases was around 3 nm, even after calcination at 773 K. XPS and thermoanalysis revealed that the particle size of the sol-gel mixed oxides was considerably larger (>10 nm), and the reduction of Bi3+ and its re-oxidation took place only in the topmost layer at ambient temperature.