Because of air quality problems, the problem Of CO2 related greenhouse gas emissions and shortage of fossil fuels, many vehicles with gaseous fuels (CNG, biogas, hydrogen, etc.) are under research and development. Such vehicles have to prove that both their exhaust emissions and their overall system emissions (including running loss) remain below certain safety limits before, they can be used in practice. This paper presents a cost-effective way of monitoring such system emissions from hydrogen or other gaseous fuel powered vehicles within an air-conditioned chassis dynamometer test cell, as commonly used for low ambient emission tests on gasoline vehicles. The only additional equipment needed is a low-concentration sensor for the gas of interest (e.g. hydrogen). The method is based on concentration measurements and a dynamic mass balance model. It is shown by a real experiment that very low emissions can be recorded. Additionally, error bounds and sensitivities on different parameters such as air exchange ratio are quantified. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.