The driving performance and emission characteristics of a 125 cc motorcycle equipped with an onboard plasma reformer for producing hydrogen-rich gas were investigated. Butane with suitable air flow rate was induced into the plasma reformer to produce hydrogen-rich gas, which was used as supplementary fuel for the internal combustion engine. The motorcycle was run under steady and transient conditions on a chassis dynamometer to assess the driving performance and exhaust emissions. Prior to the driving, the operation parameters of the plasma reformer were optimized in a series of tests and the results were an O(2)/C ratio of 0.55 and a butane supply rate of 1.16 L/min. it was shown that under a constant speed of 40 km/h, with the CO and HC emissions similar to that of the original engine, the NO(x) emission was found to be improved by 56.8%. During transient driving condition, the improvement of 16%-41% in NO(x) concentration was achieved by adding hydrogen-rich gas. The emissions of the motorcycle were also analyzed on a chassis dynamometer tracing an ECE-40 driving pattern. The NO(x) emission was improved by 34% as was the HC emission by 4.08%, although the CO emission was increased. Simultaneously, the acceleration characteristics of the vehicle were tested, and were similar under both fuelling systems. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.