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 as well as their exhaust emissions, 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. This method is based on the fact that atoms cannot vanish. Applied to a room containing a gas mixture this means that the change of mass of a gaseous matter (called gas G subsequently) inside the chamber is the difference of all mass of G flowing into the chamber and all mass of G flowing out of the chamber. This assumes that no chemical reactions of the gas in mind with other matter take place. By measuring the flow rates and concentrations of ventilation-in flow and ventilation-out flow as well as room concentration, the emissions of G of a source, i.e. the vehicle to be tested, can be calculated. These concentrations need to be measured as functions of time to be able to give values of emissions per time unit. 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