Following the recent discovery that enols are important int ediates in combustion chemistry, the possible impact of these species on atmospheric chemistry is investigated. A novel mechanism is proposed by which the reactions of enols contribute significantly towards atmospheric carboxylic acid concentrations in the gas-phase. Inclusion of ethenol in the Common Representative Intermediates CRI mechanism was found to increase predicted secondary production of formic acid considerably (∼40 pptv in the first 6 hours in a base case scenario, with typical increases of [HCOOH] ranging from 20-60 pptv over a period of 96 hours) whereas its effect on other important trace species (i.e. O3, NOx, HOx and HCHO) was negligible. Carboxylic acid concentrations are at present underestimated by major atmospheric chemical models, and these results indicate that the atmospheric transformation of enols may be an important missing secondary source term for carboxylic acids in the urban troposphere. Copyright 2007 by the American Geophysical Union.