Because of air quality problems, the problem of CO2 related greenhouse gas emissions and shortage offossil fuels, many vehicles with gaseous fuels (CNG, biogas, hydrogen etc.) are under research anddevelopment. Such vehicles have to prove that as well as their exhaust emissions, their overall systememissions (including running loss) remain below certain safety limits before they can be used inpractice. This paper presents a cost-effective way of monitoring such system emissions from hydrogenor other gaseous fuel powered vehicles within an air-conditioned chassis dynamometer test cell, ascommonly used for low ambient emission tests on gasoline vehicles. The only additional equipmentneeded is a low-concentration sensor for the gas of interest (e.g hydrogen). The method is based onconcentration measurements and a dynamic mass balance model. This method is based on the fact thatatoms cannot vanish. Applied to a room containing a gas mixture this means that the change of massof a gaseous matter (called gas G subsequently) inside the chamber is the difference of all mass of Gflowing into the chamber and all mass of G flowing out of the chamber. This assumes that no chemicalreactions of the gas in mind with other matter take place. By measuring the flow rates andconcentrations of ventilation-in flow and ventilation-out flow as well as room concentration, theemissions of G of a source, i.e. the vehicle to be tested, can be calculated. These concentrations needto be measured as functions of time to be able to give values of emissions per time unit. It is shown bya real experiment that very low emissions can be recorded. Additionally, error bounds and sensitivitieson different parameters such as air exchange ratio are quantified.