A method for the measurement of the photocatalytic activity of titanium dioxide dispersed in cementitious building materials was developed as part of the European Project PICADA (Photocatalytic Innovative Coverings Applications for Depollution Assessment). The method is based on a specially designed mixed flow reactor. It is aimed at the measuring of the photodegradation of organic compounds in air at ppb level at the surface of photocatalytic materials. The use of an actively mixed flow reactor results in a uniform concentration of reactants at the catalytic material surface at high conversion factors which also allows to measure the photocatalytic activity bypassing the limitations imposed by the concentration gradients of unmixed flow reactors. A titanium dioxide modified cementitious material was studied by applying the described method, with a benzene, toluene, ethylbenzene and o-xylene (BTEX) mixture used as organic pollutant standard. The pollutant concentrations and irradiation levels used throughout the study were comparable to those that can be found under real ambient conditions. The effects of variation of pollutant concentration, irradiation level and titanium dioxide percentage in the cementitious materials were studied. The photocatalytic activity of a pure titanium dioxide film was also measured to stand as reference benchmark. The cementitious photocatalytic material showed an interesting photocatalytic activity with linear dependence versus pollutant air concentration and irradiance. On the other hand the variation of titanium dioxide content (from 0 to 5.6% as dry powder) in the cementitious mixture showed a non linear relationship denoting a relative loss of efficiency at higher concentrations.