The system ASC carbon filter/cyanogen chloride was studied by simulation. Three filter configurations and several models, taking into account Langmuir adsorption and second-order reaction between the adsorbed toxic vapour and the active metal on the surface, were developed. The effects of axial dispersion, number of reaction units, film mass transfer units and intraparticle mass transfer resistance on the breakthrough time were studied. Simulation results show that a complex model should be used in order to predict with reasonable accuracy the protection imparted by these filters.