Concern over emissions from heavy-duty diesel engines at high altitudes prompted an investigation into the effects of increasing altitude on gaseous and particulate emissions. On behalf of the Engine Manufacturers Association, emissions from a Detroit Diesel Corporation Series 60 at local test conditions (barometer 98.9 kPa), and two simulated altitudes, Denver (82.6 kPa) and Mexico City (77.9 kPa) were examined using a special altitude simulation CVS.Transient torque output and full load steady-state torque, for this turbocharged aftercooled engine, decreased slightly with increasing altitude. Although, the DDC Series 60 compensates for variation in barometer, transient composite emissions of HC, CO, CO2, smoke, and particulate matter generally increased with increasing altitude for both transient and steady-state operation. No significant change in transient composite NOx emissions with altitude was noted during transient testing, but NOx emissions slightly decreased with increasing altitude for steady-state operation. Peak smoke opacity determined in the snap-smoke test procedure increased approximately 80 percent when apparent barometric pressure was decreased from 98.9 kPa to 77.9 kPa.HEAVY-DUTY DIESEL engines contribute to ambient air pollution at high altitude locations such as Denver and Mexico City The Engine Manufacturer's Association (EMA) sponsored this study in an effort to examine the effects of altitude on emissions. The program involved the characterization of high and low altitude heavy-duty diesel engine emissions using a special altitude simulation constant volume sampler (CVS) at “SwRI (98.9 kPa),” “Denver (82.6 kPa),” and “Mexico City (77.9 kPa) apparent barometric pressures. Regulated gaseous and particulate emissions were measured over cold- and hot-start transient tests, as well as steady-state operation. (1,2)* Smoke emissions were measured over both the FTP for smoke and the snap-smoke test procedure. (3)This paper describes the high altitude simulation CVS, the engine, test procedures, and the emission results.