New York City Department of Sanitation has operated natural gas fueled refuse haulers in a pilot study: a major goal of this study was to compare the emissions from these natural gas vehicles with their diesel counterparts. The vehicles were tandem axle trucks with GVW (gross vehicle weight) rating of 69,897 pounds. The primary use of these vehicles was for street collection and transporting the collected refuse to a landfill. West Virginia University Transportable Heavy Duty Emissions Testing Laboratories have been engaged in monitoring the tailpipe emissions from these trucks for seven-years. In the later years of testing the hydrocarbons were speciated for non-methane and methane components. Six of these vehicles employed the older technology (mechanical mixer) Cummins L-10 lean burn natural gas engines. Five trucks were equipped with electronically controlled Detroit Diesel Series 50 lean burn engines, while another five were powered by Caterpillar stoichiometric burn 3306 natural gas engines. The Caterpillar engines employed an exhaust oxygen sensor feedback and three way catalysts. Since the refuse haulers had automatic Allison transmissions, and since they were employed in stop-and-go city service, initial emissions measurements were made using the Central Business Cycle (SAE J1376) for buses, at 42,000 pound test weight. Some additional measurements were made using an ad hoc cycle that has been designed to be more representative of the real refuse hauler use that included several compaction cycles. The Cummins powered natural gas vehicles showed oxides of nitrogen and carbon monoxide emission variations typically associated with variable fuel mixer performance. In the first year of testing, the stoichiometric Caterpillar engines yielded very low emission levels, but in later years two of these refuse haulers had high carbon monoxide attributed to failure of the feedback system. For example, carbon monoxide on these two vehicles rose from 1.4 g/mile and 10 g/mile in 1995 to 144.9 g/mile and 57.8 g/mile in 1996. These stoichiometric engines were also less fuel efficient than their lean burn counterparts. The Detroit Diesel Series 50 powered refuse haulers produced high levels of oxides of nitrogen. However, it was found that changing the shifting patterns of the transmission lowered the oxides of nitrogen. All three engine types showed the potential for low emissions operation and the particulate matter reduction advantage offered by natural gas was evident from the results.