Nitrogen oxides (NOx) emissions are creating significant air quality challenges in China's megacities. Since Euro IV diesel buses equipped with selective catalyst reduction (SCR) systems failed to mitigate on-road NOx emissions as expected, real-world NOx emissions from newly introduced Euro V diesel buses, diesel hybrid and alternative fuel (e.g., natural gas) buses are of great concern to policymakers in China. In this study, NOx emissions from two Euro V diesel, two Euro IV diesel hybrid, nine compressed natural gas (CNG) and two liquefied natural gas (LNG) buses were measured on-road by using portable emission measurement systems (PEMS). The average NOx emission factor of the Euro V diesel buses was 7.5 +/- 0.1 g km(-1) for a typical driving cycle, 37% lower than the Euro IV diesel buses. However, the average brake-specific emission factor still exceeded the Euro V standard by 180%. The diesel hybrid buses had an average NOx emission factor of 4.4 +/- 1.1 g km(-1), much lower than their conventional diesel counterparts. CNG and LNG buses also had lower NOx emission factors. The average NOx emission factor of the LNG buses was 3.2 +/- 0.7 g km(-1), due to the performance of the SCR systems under higher exhaust temperatures. Furthermore, real-world NOx emission factors for all tested vehicle categories except diesel hybrids were significantly sensitive to changes of average speed. Operation of air conditioning in the bus reduced average NOx emissions by 38% for diesel hybrid buses although fuel consumption increased. These results suggest hybrid and CNG/LNG technologies are better options than the Euro V diesel engines to mitigate NOx emissions from urban buses. (C) 2014 Elsevier Ltd. All rights reserved.