Abstract: An emission-concentration monitoring system (20)includes first and second monitor stations (22,32) which are separated by a sensing space (40) along a path (28) of a moving vehicle (26). Each station has a source of electromagnetic radiation (64) which is directed through the vehicle's exhaust plume. Each station also has a set of detectors (66) which are positioned to receive the radiation and configured to measure transmittances at wavelengths which are absorbed by molecular species of exhaust plume (e.g., hydrocarbons, nitric oxide, carbon monoxide and carbon dioxide). These sensed transmittances are converted to emission concentrations by a data processor (50) and compared to a set of emission-concentration standards. The vehicle is determined to be in violation only if its emission concentrations at both the first and second monitor stations exceeds the standards.