Binary materials composed of the oxides SiO2, TiO2and N-doped TiO2and the Keggin heteropolyacid (PW12) were prepared and physicochemically characterized. They were used as catalysts and photocatalysts for the hydration of propene to 2-propanol. The characterization of the samples, particularly the electron paramagnetic resonance (EPR) spectroscopy results and the acidity properties, were useful to explain the key role played by the PW12in the composite materials in the thermal and photoassisted catalytic processes. The simultaneous pres-ence of heat and UV light improved the activity of PW12in the thermal process, and the binary materials showed better (photo)catalytic activities than that of the bare PW12in almost all cases. For the first time, this work evidenced through EPR spectroscopy that the increase of reactivity under irradiation could be attributed to the ability of photoexcited PW12to trap electrons, particularly if the PW12is supported. Moreover, the effect of water on the reactivity was also studied.