The distribution of two different phases in a mixed oxide material could be investigated through several physicochemical characterization techniques. However, the estimation of the fraction of the total surface area corresponding to each oxide is a very difficult task. In this work, we present a novel procedure for the determination of the titanium dioxide surface in titania-silica materials. This new method is based on the measurement of the phosphorus content of the mixed oxide after reaction with phenylphosphonic acid. The quantification of the TiO2 surface has permitted the comparison of the catalytic activity of different materials in processes in which titanium dioxide is the only catalytically active phase and silica behaves as an inert support, as, for instance, in photocatalytic reactions. The activity of several TiO2/SiO2 photocatalysts for cyanide and methanol photooxidation have been analysed and compared with pure TiO2 materials in terms of equal mass of semiconductor, photonic efficiency and active surface area. The results suggest the possibility of achieving surface activity rates even higher than the material Degussa P25 when using nanocrystalline titania supported on silica. (C) 2007 Elsevier B.V. All rights reserved.