Carbonyls from gasoline-powered light-duty vehicles (LDVs) and heavy-duty diesel-powered vehicles (HDDVs) operated on chassis dynamometers were measured by use of an annular denuder−quartz filter−polyurethane foam sampler with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine derivatization and chromatography−mass spectrometry analyses. Two internal standards were utilized based on carbonyl recovery: 4-fluorobenzaldehyde for <C8 carbonyls and 6-fluoro-4-chromanone for ≥C8 compounds. Gas- and particle-phase emissions for 39 aliphatic and 20 aromatic carbonyls ranged from 0.1 to 2000 µg/L of fuel for LDVs and from 1.8 to 27 000 µg/L of fuel for HDDVs. Gas-phase species accounted for 81−95% of the total carbonyls from LDVs and 86−88% from HDDVs. Particulate carbonyls emitted from a HDDV under realistic driving conditions were similar to concentrations measured in a diesel particulate matter (PM) standard reference material. Carbonyls accounted for 19% of particulate organic carbon (POC) emissions from low-emission LDVs and 37% of POC emissions from three-way catalyst-equipped LDVs. This identifies carbonyls as one of the largest classes of compounds in LDV PM emissions. The carbonyl fraction of HDDV POC was lower, 3.3−3.9% depending upon operational conditions. Partitioning analysis indicates the carbonyls had not achieved equilibrium between the gas and particle phases under the dilution factors of 126−584 used in the present study.