Fuel vapor restraint systems are used in vehicles to avoid discharge of volatile hydrocarbons from fuel tanks. Until 2020 nearly 10% of all fossil fuels shall be replaced by biofuels upon which bioethanol plays an important role. The topic of this article is the proper operation of fuel vapor restraint systems depending on the composition of bioethanol-fuel blends. As a first step, activated carbons commonly used in fuel restraint systems are tested. Received results and measured data serve as input for a mathematical model with the intention to simulate the performance of fuel vapor restraint systems depending on bioethanol-fuel composition.