A novel method determining compound-specific carbon isotopic compositions for atmospheric formaldehyde and acetaldehyde in ppb or sub-ppb levels by gas chromatography/ combustion/isotope ratio mass spectrometry (GC/C/ IRMS) is presented. Atmospheric carbonyls are collected using the conventional 2,4-dinitrophenylhydrazine (DNPH) derivatization method, and their delta13C values are calculated based on stoichiometric mass balance after measuring the carbon isotopic compositions of the carbonyl-DNPH derivatives and DNPH, respectively. Using formaldehyde, acetaldehyde, and DNPH standards with their delta13C values predetermined, the delta13C fractionation is evaluated for derivatization processes both in solutions and in simulation experiment of atmospheric sampling. In these two derivatization systems, through reduplicate delta13C analysis, good reproducibility of the derivertization process is found with an average error of less than 0.5 per thousand, and the differences between the predicted and the measured delta13C values range from -0.18 to 0.49 per thousand, indicating that the derivatization process introduces no isotopic fractionation for both formaldehyde and acetaldehyde. Thus, the delta13C values of the original underivatized carbonyls can be accurately calculated through mass balance equation. Using the method developed, preliminary tests of atmospheric formaldehyde and acetaldehyde at two urban sites were conducted and revealed significant differences of their isotopic compositions, implying possible application of the method in helping us understand the primary emission, secondary formation, or removal processes of carbonyls in the atmosphere.