The Automobile Manufacturers Association (AMA) and the Engine Manufacturers Association (EMA) contracted with Ethyl Corp. to determine the effects of humidity and temperature on HC, CO, CO2, NOx, and smoke emissions on six different manufacturers' engines run over a 13-mode cycle. Emission procedures used are similar to “California Procedures for Diesel Engines in 1973 and Subsequent Model Year Vehicles Over 6001 Pounds Gross Vehicle Weight” as adopted November 18, 1970 and amended February 17, 1971. Equations are recommended to correct NOx emissions back to standard conditions of 75 grains of H2O/lb dry air and 85 F. Humidity had no significant effect on other emissions. Temperature substantially affected the HC, CO, and smoke emissions of some engines. The correlations between four NO/NOx instruments and between two smoke measurement techniques are reported. Future work is needed to explain the different levels of NO/NOx shown by the different instruments when measuring diesel exhaust at low concentrations.