Urban air quality in California can have a large impact on the state's economy, natural and managed ecosystems, and human health and mortality. The use of alternative, low-carbon fuels is considered to be an effective measure to meet strict emissions regulations of particulate matter (PM) and oxides of nitrogen (NOx). Natural gas may be a potential alternative to conventional liquid fuels for use in automotive internal combustion engines, and can be used in fulfilling these requirements.The primary objective of this study is to evaluate the impact of varying natural gas composition on the exhaust emissions from a transit bus equipped with a 2003 Cummins C Gas Plus, lean-burn, spark-ignited natural gas engine and an oxidation catalyst while operating on the Central Business District (CBD) cycle on a chassis dynamometer. The vehicle was tested on five different fuel gas blends with varying compositions of light hydrocarbon species and inerts, resulting in different properties in terms of methane number (a measure of fuel knock resistance) and Wobbe number (a measure of fuel interchangeability). For this study, emissions of NOx, total hydrocarbons (THC), nonmethane hydrocarbons (NMHC), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂) emissions, and PM, as well as fuel economy were measured. The unregulated exhaust emissions were characterized by determining formaldehyde, acetaldehyde, ammonia, and total concentration and sizing of ultrafine particles. The results showed that natural gas composition had a strong influence on a number of emission components. Blends with higher methane contents showed lower NOx, CO, and NMHC, but higher THC, CH₄, and formaldehyde emissions. PM, CO₂, and NH₃ emissions and energy equivalent fuel economy did not show consistent trends between the fuels tested.