Exhaust gas odor threshold dilution ratios were measured during idle operation of a single-cylinder 4-stroke cycle diesel engine using n-heptane as a fuel. Odor threshold dilution ratios were determined by a single panelist using a sample presentation termed the Sniff-Mask technique.The effect of various changes in the intake atmosphere composition on odor thresholds was determined. These composition changes fall into two general classes: 1. Substitution of the inert gases argon, helium, and carbon dioxide for the nitrogen contained in the normal intake air. 2. Addition of the inert gases argon, nitrogen, and carbon dioxide to the normal intake air.The substitutions of argon and helium produced a 15 fold reduction in the exhaust odor thresholds, while the substitution of carbon dioxide increased the odor threshold by a factor of 4. Directionally similar results were found for the addition experiments.The odor results could not be explained by expected changes in the diffusional processes, in the fuel spray patterns, or in the cycle temperatures.A good correlation was found between the measured exhaust odor thresholds and the lean flammability limits of premixed flames determined in the intake atmospheres used. The correlation between exhaust odor thresholds and the lean flammability limits suggests that diesel odor is caused by odorous products of partial oxidation that are produced in regions of fuel/oxidizer mixtures too lean to burn at the start of combustion.