Selective filtration is one of the unique properties of cellulose acetate filters. Previous studies have shown that the amount of moisture in the filter enhances this filtration property. Additionally, increases in the ventilation level of a filter have been shown to reduce the tar delivery and affect a filter's ability to remove phenol. This study explored the influence of water in the upstream and downstream segments of the filter on the selective filtration of phenol. Experimental cigarettes were prepared with ventilation levels ranging from 0-75%, with 7% triacetin, and tested under ISO and aggressive smoking conditions. The filters were cut into upstream and downstrearm segments and analyzed separately. Nicotine, water, triacetin, and phenolics were measured in the Cambridge pads and in both segments of the filters. Filtration efficients for both segments was assessed by a UV spectrophotometric technique. Under ISO conditions, the results show that the removal efficiencies for tar, nicotine, and phenol in the upstream segments of the filters increased with increasing ventilation. Although there is a corresponding decrease in water content of the filter with increasing ventilation level, more water is retained in the upstream segment, which also shows greater selectivity for phenol. These results support the role of water in enhancing the selective removal of phenol by cellulose acetate filters.