A proton transfer reaction mass spectrometer (PTR-MS) was redesigned and deployed to monitor selected hydrocarbon emissions from in-use vehicles as part of the Mexico City Metropolitan Area (MCMA) 2003 field campaign. This modified PTR-MS instrument provides the necessary time response (<2 s total cycle time) and sensitivity to monitor the rapidly changing hydrocarbon concentrations, within intercepted dilute exhaust emission plumes. Selected hydrocarbons including methanol, acetaldehyde, acetone, methyl tertiary butyl ether (MTBE), benzene and toluene were among the vehicle exhaust emission components monitored. A comparison with samples collected in canisters and analyzed by gas chromatography provides validation to the interpretation of the ion assignments and the concentrations derived using the PTR-MS. The simultaneous detection of multiple hydrocarbons in dilute vehicle exhaust plumes provides a valuable tool to study the impact of driving behavior on the exhaust gas emissions.