2-Hydroxy-3-methyl-2-cyclopenten-1-one-3-methyl-14C (1-3-methyl-14C) was synthesized and employed as an internal standard for the quantitative analysis of 1 and related alkyl-2-hydroxy-2-cyclopenten-1-ones in cigarette smoke. The mainstream smoke of a typical U.S. blended 85-mm nonfilter cigarette contained 52 ± 3 µg of 1 as well as 3,5-dimethyl-2-hydroxy-2-cyclopenten-1-one (9.9 µg), 3,4-dimethyl-2-hydroxy-2-cyclopenten-1-one (2.1 µg), and 3-ethyl-2-hydroxy-2-cyclopenten-1-one (24 µg). The level of 1 in cigarette smoke was selectively reduced by cellulose acetate filter tips. The smoke of cigarettes made from Bright tobacco contained more 1 than observed in the smoke of Burley or Oriental cigarettes. Levels of 1 in smoke were reduced by extraction of tobacco with hexane-ethanol or by use of tobacco from the bottom stalk position. A correlation between concentrations of 1 and catechol in cigarette smoke was observed, indicating common leaf precursors for these two components.