This study investigated the impacts of varying natural gas composition on the exhaust emissions from different technology transit buses. For this study, two CNG (compressed natural gas) buses equipped with lean burn combustion and OCs (oxidation catalysts), and one stoichiometric CNG bus equipped with a TWC (three-way catalyst) and EGR (exhaust gas recirculation) were tested on a chassis dynamometer over the CBD (Central Business District) cycle on six different gas blends each. The gases represented a range of compositions from gases with high levels of methane and correspondingly lower energy contents/WN (Wobbe number) to gases with higher levels of heavier hydrocarbons and correspondingly higher energy contents/WN. For the lean burn buses, gases with low methane contents exhibited higher NOx (nitrogen oxides) (19%-53%) and NMHC (non-methane hydrocarbon) (39%-102%) emissions, but lower emissions of THC (total hydrocarbon) (9%-24%), CH4 (methane) (23%-33%), and formaldehyde emissions (14%-45%). The stoichiometric engine bus with a TWC showed significantly reduced NOx and THC emissions compared to the lean burn buses, but did show higher levels of CO (carbon monoxide) and NH3 (ammonia). PM (particulate matter) mass emissions did not show any fuel effects, while PN (particle number) emissions exhibited some reductions for the higher WN gases. (C) 2013 Elsevier Ltd. All rights reserved.