The U.S. Bureau of Mines is conducting laboratory evaluations of the effectiveness of oxidation catalytic converters (OCC's) for application on diesel engines used in underground mines. The composition of the exhaust emitted from a diesel engine depends critically upon the fuel-to-air ratio (F/A). The F/A changes significantly with altitude, requiring regulatory derating of engines used at altitudes above 0.30 km (1000 ft). It is important to evaluate exhaust control devices for application on mining vehicles at various altitudes because mines are located at a wide range of elevations. The objective of this work is to determine the characteristics and performance of an OCC at various altitudes as well as investigate an altitude simulation technique used in a laboratory setting.The efficiency of an Engelhard Corp.* PTX Ultra 10-DVC OCC for removing carbon monoxide (CO) and hydrocarbons measured by flame ionization detection (FID-HC) from the exhaust was determined for simulated altitudes ranging from 0.61 km (2000 ft) below to 2.74 km (9000 ft) above sea level. Altitudes were simulated by controlling the pressures at the intake and exhaust manifolds. Tests were conducted at a constant engine speed over a F/A range from 0.01 to 0.05 by changing the load on the engine, thus varying the exhaust temperature.Results showed that the lightoff temperature (T50) of the OCC increased systematically with simulated altitude for both CO and FID-HC. For FID-HC, T50 started at 260 °C at 0.61 km below sea level and reached 309 °C at 2.74 km above. For CO, T50 started at 230 °C and increased to 292 °C.The maximum removal efficiency also changed systematically with altitude, but for CO and FID-HC the direction of change was reversed. For FID-HC the maximum efficiency decreased with increasing altitude; for CO it increased. This trend was corroborated using a Fourier Transfer-Infrared (FT-IR) gas analyzer. This unexpected reversal in maximum removal efficiency between HC and CO may lead to additional insight into the mechanisms for removal of these components by the OCC.