A program was conducted to determine the effect of temperature and humidity on exhaust emissions from automotive engines. The objective was to determine if the effects were of sufficient magnitude to require the application of correction factors to measured exhaust emissions to standard humidity and temperature values. Both American and foreign-made vehicles were tested at 20 combinations of ambient temperature and humidity.The effect of temperature and humidity was found to be both unpredictable and of little significance for hydrocarbon and carbon monoxide emissions. No correction factors were developed for these exhaust gas constituents.The effect of temperature was found to be of little significance for oxides of nitrogen. However, humidity effects were found to be significant and predictable for oxides of nitrogen. A correction factor was developed to correct oxides of nitrogen to a standard humidity value and was subsequently incorporated into the federal test procedure for determining automotive exhaust emissions.