A method for the direct measurement of CO diffusion through cigarette paper was developed. Diffusing gases from a lit cigarette were trapped in a plastic sample bag and subsequently analyzed on a non-dispersive infrared analyzer (NDIR) having a 0-2% CO range. Carbon monoxide diffusion measurements were made on non-filter cigarettes equipped with air-dilution mouthpieces. Mainstream CO, mouthend dilution, and total dilution data were also obtained. It was found that on increasing mouthend dilution to 77%, CO diffusion increased to 90% of the CO initially in the tobacco rod. The diffusion data were well fit by an exponential diffusion equation which included a wrapper dilution term in addition to the tobacco rod radius and length and linear smoke velocity. It was found that cigarette paper permeability over the range of 28-157 cm/min does not affect the extent of CO diffusion. Sidestream CO was determined by completely encapsulating a cigarette in a glass envelope. Air was fed to the envelope at a rate sufficient to maintain a correct puff count. The effluent was analyzed for CO with an on-line NDIR. It was found that while increased mouthend dilution resulted in a small increase in sidestream CO, the total CO produced was reduced by up to 20%.