Mathematical modelling, on the basis of the Langmuir-Hinshelwood equation, of a photocatalytic reaction in a static reactor, taking into account in the mass balance the adsorption on the irradiated parts of the catalyst, dark parts of the catalyst and support, reveals that an increase in the surface area of the dark catalyst results in prolonged kinetic curves. If the adsorption on the irradiated and dark parts of the catalyst is ignored in the mass balance, errors can occur in the kinetic parameters calculated from the concentration-time curves. An increase in the surface area of the dark catalyst facilitates the removal of contaminants from the gas or liquid phase to below the threshold permitted concentration (TPC), if TPC corresponds to the surface coverage of the illuminated catalyst with contaminant of higher than 0.37.