Photocatalytic materials for air depollution have received increasing attention in the last years, particularly for outdoor applications where the sun is used as freely accessible UV-A source. The naturally available irradiance at an outdoor catalytic material surface is, however, extremely variable according to the time of the day, the season and the local weather. The relationship between irradiance and activity is therefore of great importance in the characterization of a photocatalytic depolluting material. This work illustrates the investigation carried out on the toluene photodegradation activity of a cementitious photocatalytic material versus the UV-A irradiance in the 100-850 mu W cm(-2) range. The photoactivity measurements were carried out using a stirred flow reactor. A specific successive approximation method was developed in order to bring all activity measurements to the same predefined pollutant concentration ensuring uniform conditions across the experiment. The study was repeated at four different pollutant concentrations lying between 0.25 and 1.0 mu mol m(-3), showing evidences of a non-linear behaviour at low irradiances. A correction method for the irradiance measurement is also described, allowing the adjustment for the error caused by the specific spectral sensitivity of the utilized radiometric sensor. (C) 2011 Elsevier B.V. All rights reserved.