The present study aimed at the quantification of primary r-hotocatalytic processes occurring over irradiated semiconductors. The photodegradation of HCOOH was selected as a probe reaction since it follows a single-stage oxidation mechanism (yielding CO2 and H2O), thus not relying on the formation of stable intermediate products or multiple reactions. This reaction was used to determine lumped kinetic parameters of W-assisted photo-catalysis in semiconductor slurries. A modified Fricke dosimeter (Fe2+ --> Fe3+) was utilized to determine the exact concentrations of reactive oxygen species in the slurry, such as (OH)-O-., HO2., O-.(2)-, and H2O2. These experiments yielded maxima in the concentrations of these species during the first few minutes of reaction with the subsequent attainment of a steady state. The rate of generation of reactive oxygen species characterizes the true oxidative power of photocatalysts, and it was found to increase with the catalyst concentration and reach a plateau value in the vicinity of 0.25 g/l of catalyst. This generation rate follows the pattern Ishihara ST21 approximate to Hombikat UV100 approximate to Degussa P25 > Aldrich anatase; its value for Aldrich anatase is approximately one-third of that for the other catalysts. The effect of direct oxidation by valence-band holes was studied using the oxidation of NO2- to NO3- and proved to be minimal. From the kinetic data of the photodegradation of formic acid, rates of primary photocatalytic processes, such as radical generation (R-gen) and electron-hole recombination (R-rec), were determined with our methodology. In particular R-ec was found to be inversely proportional to the BET surface area of the catalyst following the sequence Aldrich anatase > Degussa P25 > Ishihara ST21 approximate to Hombikat UV100. Using the present methodology, the quantum yield of a particular photocatalyst can be predicted satisfactorily from its radical generation rate. It should be noted that such quantum yield depends on the catalyst concentration and reactor setup to a much lesser extent than the traditionally used quantum yield for phenol photodegradation, since the latter pro-ess is accompanied by multiple secondary oxidation reactions. Thus, the method proposed provides a convenient procedure for a thorough study on semiconductor photocatalysis and can serve as a common means of photocatalyst characterization.