Abstract: To remove the aldehydes from tobacco smoke by activated carbon used in a cigarette filter, the effects of activated carbon surface area, particle size, pore size and distribution, and surface chemistry have been systematically evaluated for the filtration of aldehydes in mainstream cigarette smoke. A puff-by-puff analysis on selected aldehydes was also carried out to further illustrate the dynamic interaction between activated carbon and the smoke vapours. The aldehydes in smoke of cigarette with filter containing an amine functionalised ion exchange resin, Diaion^® CR20, was determined to explore the effect of surface chemistry further. The results showed that: Firstly, a change in the activated carbon particle size distribution from 500-600 to 300-400 m, resulted in a small improvement in the filtration performance. Secondly, an increase in activated carbon micropore volume and surface area resulted (with the exception of acetaldehyde) in a higher aldehyde filtration efficiency. Thirdly, the filtration efficiency was further improved using a combination of micropores and mesopores (transport pores). Fourthly, the activated carbon surface chemistry was modified by increasing the nitrogen content through a synthesis with ethlenediamine, which resulted in an improvement in the selectivity of formaldehyde from the cigarette smoke; this was independent of the porosity, implying that chemisorption played a significant role in addition to physisorption. Fifthly, the position of the activated carbon in the cigarette filter (at a constant weight of 60 mg) showed a small improvement in the aldehyde filtration efficiency under ISO smoking conditions as the activated carbon loading per unit length in the filter rod was increased from 2.5 to 5.0 mg/mm. However under a more intensive smoking regime, there was no observable difference in the filtration efficiency. Finally, an amine functionalised CR20 showed a greater selectivity than that of activated carbon, showing the importance of both physisorption and chemisorption mechanisms in minimizing mainstream smoke aldehyde yields.