The use of a chemically stable TiO 2 sol as an actinometer for UV radiation was investigated. The sol produces a visible color change under UV radiation. It can be regenerated to its original transparency upon exposure to oxygen. The actinometer can be used to compare the strengths of different light sources for a wide range of photoinduced chemical processes involving TiO 2 particles such as photodegradation of industrial waste or organic synthesis on TiO 2, photoinduced deterioration of TiO 2-containing materials, and the effectiveness of a TiO 2-based sunscreen product. Other potential applications include γ-ray radiation monitoring, oxygen detection, and oxygen permeability measurement for polymer films.