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Emissions of particulate matter (PM) and oxides of nitrogen (NOx) from heavy-duty engines used in road vehicles and mobile equipment are of critical concern to air quality planners. Accurate measurement of these emissions is needed both for emission inventory purposes and for the development and validation of emission control measures. These emissions cannot adequately be predicted from laboratory tests on new engines. This paper reports the development of the Ride-Along Vehicle Emission Measurement (RAVEM) system. Based on the constant volume sampling (CVS) principle, the RAVEM system can accurately and repeatably measure mass emissions of NOx, CO2, and PM from an engine exhaust pipe while “riding along” on the vehicle or equipment under test. In most cases, this can be done without removing the vehicle from service. RAVEM test results have been shown to correlate very closely with those of conventional CVS emission laboratories.