Time-resolved sampling was used to measure hydrocarbon concentration profiles in the exhaust of a small, high-speed, direct-injection diesel engine in an effort to determine significant sources of hydrocarbon emissions. The engine was a single-cylinder design with a 0.53-L displacement and a swirl-supported, high-pressure-injection combustion system. In the past, most sampling work performed on diesel engines has been done in the cylinder and has focused on either oxides of nitrogen or soot emissions. The exhaust sampling done in this study provided a simple yet informative means of studying hydrocarbon emissions. Results from this study for wide ranges of engine speed and air-fuel ratio showed a single peak in the hydrocarbon concentration profile as a function of crank angle. Phasing of the hydrocarbon concentration peak generally coincided with the blowdown period of the exhaust event. Because the mass flow rate of the exhaust is high during blowdown, the contribution of unburned hydrocarbons exhausted during this period to total hydrocarbon mass emissions of the engine was significant; from 36 to 60 percent of the total HC mass emission, depending on air-fuel ratio. From these results it was concluded that, except at light load, fuel issuing from the sac volume of the injector late in the cycle was a significant source of unburned hydrocarbons in the exhaust.