A portable remote sensing system capable of real-time detection of particulate emissions from heavy-duty vehicles is being developed. The remote sensing technique employs optical extinction to measure the concentration of the carbonaceous fraction of the particulate matter and the CO2 in the exhaust plumes from individual vehicles. From these two measurements, instantaneous emission indices giving the mass of soot emitted to the amount of fuel burned by the vehicle at the given operating condition can be computed. Two tests designed to characterize the accuracy of the technique were conducted: 1) measurements through an exhaust plume from a laboratory diesel using a sequence of steady-state operating conditions; and 2) measurements of transient emissions from full size diesel-powered buses whose emissions were simultaneously measured in a full-scale dilution tunnel. In both cases, the remote sensing measurements were in good agreement with those obtained by conventional methods. Applications to actual on-road measurements are also discussed.