The present paper reports additional results obtained with a method already proposed and used to measure absorbed and reflected photons during the occurrence of heterogeneous process carried out in the presence of a polycrystalline semiconductor solid suspended in aqueous solution. The limit of validity of the method is discussed and the influence of various parameters is reported. Moreover, the quantum yield for a probe photoreaction, i.e. 4-nitrophenol photo-oxidation carried out in aqueous suspensions containing various amounts of polycrystalline TiO2 (anatase) of different particle sizes, was determined by using monochromatic (λ=366 nm) or polychromatic light (λ=300–400 nm). The results indicated that the quantum yield was higher when light in the range 300–400 nm was used.