filters remove both particles and condensable vapours from tobacco smoke aerosols. The particulate contribution to filtration can be isolated by allowing the smoke aerosol to attain thermal equilibrium before it is introduced to the filter. Such experiments show the effects of filter length, filter pressure drop, and aerosol flow rate on particulate filtration. The relative roles of diffusion, impaction, and interception on particle removal have been examined. The extent to which vapour condensation occurs in filters has been found to be dependent on the length to which the tobacco rod has been smoked and independent of the pressure drop of the filter.