Diffusion of motor vehicle exhaust fumes is a function of not only the type of fuel and vehicle age but also of the vehicle movement, atmospheric wind, turbulence and thermal stability.Two methods are used to examine these effects on the diffusion of the exhaust fumes: direct probing by means of road tests using a full-scale system and wind tunnel tests using a model.Field tests have been conducted at the Meteorological Field Station at Woodbridge. The tests include a common stationwagon. A meteorological instrumented tower was used at the Atmosphere Volume 12 Number 2 1974