A study of exhaust gas composition at various engine speeds, loads, and cooling water temperatures was made on an International Harvester UD-6 Diesel Engine. Simultaneously, smoke density was measured with a smokemeter, the principle of which was based on the obstruction of light. Exhaust gases were analyzed with a Fisher Precision Gas Analyzer. The components analyzed were carbon dioxide, oxygen, illuminants, carbon monoxide, hydrogen, and methane. The results of this study have led the investigators to the following conclusions. 1. The exhaust gas composition was chiefly a function of fuel-air ratio. 2. The effect of cooling water temperature on exhaust gas composition was not conclusive over the range of temperature investigated. (120 to 160 degrees Fahrenheit) 3. The so-called “chilling of direct oxidation reactions” were experienced at maximum brake horsepower check. 4. Smoke density increased with an increase in fuel-air ratio at ratios above .04. The cooling temperature effect was negligible. 5. There was a relationship between smoke density and free, or unburned carbon in the exhaust. 6. A higher precision method of gas analysis would be required for any study of the mechanics of combustion inside the combustion chamber. / Master of Science