Real time measurements of air toxic emissions were made from vehicles undergoing transient cycles on a dynamometer. Resonance-enhanced multi-photon ionization (REMPI) with time of flight mass spectrometry (TOFMS) was used to characterize rapid responses in trace aromatic emissions, including mobile source air toxics, to changes in driving cycles. Concurrent sampling of PM, CO, and CO2, with vehicle parameters of horsepower, velocity, and total particle density allowed the air toxic emissions to be related to operating performance. Two models of the U.S. Army High Mobility Multi-purpose Wheeled Vehicle (HMMWV), a diesel (M1097) and a turbocharged diesel version (M1114), were tested. Emissions were measured while running on a dynamometer-based roadway simulator under the West Virginia and EPA Federal Highway Economy Test cycles and under steady state, constant velocity conditions. Emission factors were determined for criteria pollutants such as CO and CO2, as well as for aromatic air toxics including benzene, toluene, ethylbenzene, and xylenes, as well as small polycyclic aromatic hydrocarbons such as naphthalene and methylnaphthalene. The combined dynamometer cycle and REMPI-TOFMS system relative accuracy (triplicate) varied from 15% to 79% for benzene, 31% to 109% for naphthalene, and 47% to 214% for 2-methylnaphthalene. CO and CO2 emissions were found to poorly correlate with benzene, naphthalene, and 2-methylnaphthalene. The fast time response of REMPI-TOFMS documented the compound-specific trace concentrations during cold startups of the M1097, showing benzene at ~230 ppb but then rapidly (within 60 s) tailing off to a steady state level around 50 ppb. This work highlights the ability of REMPI-TOFMS to measure trace air toxics in real time, following rapid changes in emissions during roadway cycles that cannot be identified using typical long term (minutes) integrated sampling methods.