Transit agencies across the United States operate bus fleets primarily powered by diesel, natural gas, and hybrid drive systems. Passenger loading affects the power demanded from the engine, which in turn affects distance-specific emissions and fuel consumption. Analysis shows that the nature of bus activity, taking into account the idle time, tire rolling resistance, wind drag, and acceleration energy, influences the way in which passenger load impacts emissions. Emissions performance and fuel consumption from diesel and natural gas powered buses were characterized by the West Virginia University (WVU) Transportable Emissions Testing Laboratory. A comparison matrix for all three bus technologies included three common driving cycles (the Braunschweig Cycle, the OCTA Cycle, and the ADEME-RATP Paris Cycle). Each bus was tested at three different passenger loading conditions (empty weight, half weight, and full weight). Carbon dioxide (CO2) from a John Deere powered natural gas bus was higher at full weight than that at empty test weight. However, carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), and particulate matter (PM) from this bus showed a mixed trend. The bus, on average, consumed about 9% more fuel at full weight than that at empty weight. NOx emissions from this bus varied considerably on all cycles at all weights. NOx emissions from a Cummins powered natural gas bus were higher at full weight than those at empty weight on all cycles. However, no difference in NOx emissions at half and full weights was observed. CO2 emissions from the Cummins powered bus increased with increasing test weights. CO2 also increased with increasing weights on a retrofitted diesel bus. NOx from the retrofitted diesel bus was higher at full weight than that at empty weight. However, mixed patterns were observed for CO, HC, and PM emissions from this bus. PM and CO values from these buses were low and it was difficult to quantify the weight effects trends.