Three fuels (propane, isooctane and toluene/isooctane) are used for the study of the formation of organic acids from their flames. Four organic acids have been found in the combustion products: formic, acetic, propionic and isovaleric acid. These acids are formed very quickly; their concentration then generally increases to reach a maximum value, and then decreases to zero. Toluene enhances the formation of organic acids comparing to propane and isooctane. The concentration of these acids depends strongly on the air/fuel equivalence ratio. Some correlations are found between the concentration of the acids and some alcohols or aldehydes. These results are in accordance with those presented in the case of internal combustion engines. (C) 2004 Elsevier Ltd. All rights reserved.