WE wish to comment on an omission of generality in the interpretation of data obtained by Green and Sugden1 concerning ionization in flames which contain hydrocarbons. These data form the quantitative basis for the reaction which is usually accepted to be responsible for CHO+ in hydrocarbon flames2–7. With premixed flames of hydrogen, oxygen and nitrogen at atmospheric pressure, and a < 1 per cent admixture of C2H2, Green and Sugden analysed their data according to the general mechanism This mechanism requires that [AH+] ∝ [H3O+]2max; the only ion observed which fits this criterion for a primary ion was found to be CHO+. Furthermore, the rate of ion formation was found to be proportional to [C2H2]0, thus B was assumed to be the only species containing carbon, C being a normal constituent of hydrogen–oxygen flames such as H, OH or O. Of the three possibilities for the formation of CHO+, C + OH, CO + H, and CH + O, the last was chosen as the most favourable.