The report presents the results of a test program to evaluate the effect of ambient temperature and humidity conditions on automobile exhaust emission levels. Concentration measurements of unburned hydrocarbons, carbon monoxide, carbon dioxide, and nitrogen oxide, as well as mass measurements of unburned hydrocarbons, carbon monoxide, and oxides of nitrogen were made at 20 different combinations of ambient temperature and humidity conditions. Five American-made vehicles and three foreign vehicles were evaluated at each of the 20 different test conditions. The test conditions were selected to represent the range of temperature and humidity values encountered at most exhaust emission testing facilities in the continental United States. (Modified author abstract)