An ion chromatographic procedure was applied to the determination of organic acids in tobacco smoke. Cigarettes on the market were smoked under the standard conditions (1 puff/min, 2sec/puff, 35ml/puff, 30mm in butt length), and the smoke was collected in a cold trap cooled with dry ice _ methanol. The smoke condensate was extracted with deionized water, and then the aqueous extracts were passed through a 0.5 microm membrane filter. An ion chromatogram of organic acids in the smoke was obtained, using a phosphoric acid eluent. Acids in the smoke were identified by comparing their ion chromatographic retention times with those of authentic compounds. Sixteen carboxylic acids were identified positively or tentatively in this study. Semi-quantitative estimation of some of these acids was carried out for cigarettes on the market. Formic and acetic acid occupied from 46 to 57% of the total amount of acids determined semi-quantitatively in the smoke. The efficiency of tobacco filter materials for organic acid retention was examined for cellulose acetate and cellulose filters were higher in formic, acetic, and glycolic acid retention than polypropylene. The polypropylene filter had relatively low filtration efficiency for almost all carboxylic acids in the smoke.