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Three widely documented methods for the analysis of aliphatic aldehydes in air, i.e., chromotropic acid, 3-methyl-2-benzothiazolone hydrazone (MBTH) and 2, 4-dinitrophenyl hydrazone (DNPH), and a modified version of the MBTH method are frequently used for the analysis of aldehydes in diluted diesel exhaust. In order to assess their relative accuracy for analysis of aldehydes in such a matrix, a side-by-side comparison of the methods was conducted. The equivalent accuracy of the chromotropic acid, MBTH and DNPH methods for analysis of formaldehyde in a clean air matrix was confirmed and a negative bias in the MBTH method as a result of SO2 interference was documented. A comparison of the concentrations of formaldehyde and aliphatic aldehydes in diluted diesel exhaust measured by the four methods indicates that significant differences exist between several of them.