What is disclosed is a system and method for image-based determination of concentration of CO and CO2 in a vehicle's exhaust gas in an emissions testing environment. In one embodiment, the present method involves receiving an IR image of the exhaust plume of a motor vehicle intended to be tested for CO and CO2 concentrations. The IR image has been captured using a mid-wave infrared camera with at least one optical filter tuned to the infrared absorption band of CO and CO2. The images are pre-processed to isolate pixels which contain the exhaust plume. The intensity values of pixels in those isolated regions are normalized and concentrations of CO and CO2 are determined via a calibration curve which relates pixel intensities to concentrations. The concentrations are compared to an emissions standard set for the vehicle to determine whether the vehicle is a gross polluter.