As smoke is drawn out of the cigarette burning zone and through the tobacco rod during a puff the concentration of light gases such as carbon monoxide falls due to diffusion out of the rod and to dilution with air drawn in through the paper. Earlier work has concentrated on the dilution effects and diffusion through the cigarette paper. Diffusion through the tobacco bed is equally important and this paper covers a detailed study of the diffusion of carbon monoxide through tobacco and filter rods. The total diffusion coefficient was obtained iteratively by comparing experimental results with predictions of a mathematical model of the mass transfer and gas flow processes occurring in the porous rod. The total diffusion coefficient of carbon monoxide/nitrogen through the tobacco rods increases linearly as the mean gas flow rate through the tobacco rod increases. This is caused by a contribution of convective dispersion to the total diffusion, due to the bulk flow of the gas, as observed in other studies on porous beds of sand particles. The molecular diffusion and transverse convective despersion components of the total diffusion coefficient have been calculated. The molecular diffusive component is related to the porosity (void fraction) and tortuosity of the porous material. The calculated tortuosity factors are in the range 1.1 to 1.3 for tobacco beds, and 1.9 for filter beds. The variation of the convective despersion component of total diffusion with total particle diameter is qualitatively (but not quantitatively) similar to that predicted by equations derived from studies on sand beds.