The emission factors of carbonyls from tested diesel vehicles are shown in Table 2, together with literature results. Total average carbonyl emission factor was 97.2 mg/km or 398.3 mg/L. For VECC cycle and C-WTVC cycle, the emission factors were 109.1 and 69.6 mg/km, respectively. Formaldehyde, acetaldehyde, acetone, prop onaldehyde and methacrolein were the dominant species. Low molecular weight species (< C5) accounted for 40% of total carbonyls. As shown in Table 2, the emission factors of diesel vehicles in this study were higher than that of light-duty diesel vehicles measured in Taiwan (Tsai et al., 2012), but lower than that of HDDVs results by Schauer et al. (1999).