The GC determination of aldehydes and ketones in stack gas is reported. The stack gas is collected with the dust sampler by isokinetic sampling technique, 2,4-dinitrophenylhydrazine in acidic solution is used to absorb aldehydes and ketones in stack gas specifically, to form their hydrazones, which are determined by GC with a capillary column of SE-54. The GC system can separate nine hydrazones of carbonyls interested with constant recovery rates of 94%-97%, giving lowest detectable limits of 0.05-1.0 ng. The standard gas of carbonyls are determined by this method with relative errors of -5%--10%, percent coefficients of variation within 5%, and spiking recovery rates of 93%-97%. As an application of the method, the carbonyl pollutants are determined in stack gas samples in a chemical plant.