A method for the sampling and determination of exhaust aldehydes and ketones is described. The procedure consists of absorbing and converting these compounds to the solid 2, 4 di-nitrophenylhydrazone derivatives. Results are reported as total aldehydes and/or the derivatives separated into individual, identifiable components by chromatographic techniques.Exhaust emission data employing this procedure are presented for a limited number of vehicles with and without exhaust control systems. Total aldehyde levels (as formaldehyde) were found to range from about 20 to over several hundred parts per million depending on the mode of operation and the adjustment of such variables as air-fuel ratio, spark timing, and exhaust emission control devices. Effects of these variables on aldehyde emissions are discussed.The relationship of the chemical structure of inducted fuel to aldehyde emissions is also touched upon. The amount of individual aldehydes was found to be related to the parent fuel to a considerable extent for pure individual hydrocarbon fuels. This relationship is greatly diminished, however, within the design limitations of current full boiling practical gasolines.