Techniques have been developed to sample and speciate dilute heavy duty diesel exhaust to determine the specific reactivities and the ozone forming potential. While the Auto/Oil Air Quality Improvement Research Program (AQIRP) has conducted a comprehensive investigation to develop data on potential improvements in vehicle emissions and air quality from reformulated gasoline and various other alternative fuels. However, the development of sampling protocols and speciation of heavy duty diesel exhaust is still in its infancy [1, 2, 3, 4, 5 and 6]. This paper focuses on the first phase of the heavy duty diesel speciation program, that involves the development of a unique set of sampling protocols for the gas phase, semi-volatile and particulate matter from the exhaust of engines operating on different types of diesel fuel. Effects of sampling trains, sampling temperatures, semi-volatile adsorbents and driving cycles are being investigated.Dilute exhaust is sampled from a total exhaust double dilution tunnel system via two separate streams, viz. a “cold” stream with a filter face temperature of less than 52°C (125°F) and a “hot” stream at temperatures that will be varied from 93°C (200°F) to 204°C (400°F). The cold line is employed to collect particulate matter on 70 mm fluorocarbon coated glass fiber filters and collect gas samples in a Tedlar bag. The “hot stream” uses the 70 mm filters along with a bed of adsorbent contained in a custom built stainless steel cartridge followed by a Tedlar bag to collect the gases. All of the collected samples were analyzed by gas chromatography after conditioning and chemical extraction. The preliminary results of some steady state tests will be discussed as per the identification of compounds with a high maximum incremental reactivity (MIR) value or a high concentration.