Individual aldehyde (and acetone) emissions were measured from the exhaust gas of a premixed multicylinder spark ignition engine fueled with Indolene and blends of Indolene and either methanol or ethanol. The engine was operated at constant speed (2000 RPM) and MBT spark advance with fuel-air equivalence ratios (Φ) of 0.96, 0.90 and 0.82. During operation at Φ = 0.82, the engine experienced lean-limit misfiring.The DNPH method with a gas chromatographic finish was employed to obtain exhaust gas concentrations of aldehydes and acetone. Also, the methods used in the past for measuring engine exhaust aldehyde and acetone data were compared to each other and briefly discussed.Use of the alcohol blends increased the total aldehyde emission level. Formaldehyde was the largest component, exhibiting a continual increase with increasing alcohol blend level. Acetaldehyde, while exhibiting a small decrease with increasing methanol percentage, showed a marked increase when the ethanol blend was used.Acetone was the only specie measured with the DNPH method that showed a marked increase when the engine was operated in the lean misfire region (Φ = 0.82).The C3 aldehydes were successfully separated and quantified. It was found that acrolein emissions did not increase when the blends were used; in fact, a slight decrease was observed. Propionaldehyde exhibited a decrease when the methanol blends were used.Samples taken in the exhaust manifold, upstream of an oxidizing catalyst, and downstream of the catalyst, indicated that aldehydes and acetone are partially destroyed in the exhaust system and virtually completely destroyed by the catalyst.