This paper reports results from a quantitative study on the effects of diesel piston temperature and fuel sulfur on piston deposits. Tests were conducted in a fourcycle, turbocharged single-cylinder engine using uncompounded oils and distillate fuel.This fundamental study demonstrated that at constant exhaust smoke, piston temperature between 200°C to 260°C is the primary factor in controlling piston top groove deposit formation. The activation energy for this process was determined to be 12 kcal/mole, which is within the normal range of chemical reactions. The rate of top groove deposit formation doubled for every 30°C rise in piston temperature. Piston temperature also increased total piston deposits on all grooves and lands.Fuel sulfur in the range of 0 to 1.0 wt % had no effect on top groove deposit formation, although it did increase lower groove and lower land deposits.Crankcase oil oxidation appeared to correlate with piston temperature. The activation energy for this process was determined to be 5 kcal/mole. The rate of crankcase oil oxidation doubled for each 67°C increase in piston top groove temperature.