Variations in atmospheric humidity, temperature, and barometric pressure substantially affect almost all engine mass and volume exhaust emissions. Emission measurements were conducted on seven engines in accordance with the Gasoline Fueled Heavy-Duty Engine procedures in the 1970 Federa Register. Equations with satisfactory correlation were derived to adjust NO mass and volume emissions taken at test conditions to a standard condition of 75 grains of moisture. Less satisfactory equations were derived to similarly correct HC emissions for moisture effects and to correct CO emissions to standard conditions of 90 F and 29.92 in. Hg. Unfortunately, no other equations could be derived to describe the behavior of all seven engines because engine-to-engine variation was too great. Emissions from different engines are affected differently at various atmospheric conditions. Therefore, accurate emission testing on some engines can only be accomplished by controlling inlet air temperature and by testing on days of similar barometric pressure.