This paper compares the current taxation of diesel and gasoline cars in Belgium with the guidelines for optimal taxation. We find that diesel cars are still taxed much less than gasoline cars, resulting in a dominant market share for diesel cars in the car stock. If the fuel tax is the main instrument to control for externalities and generate revenues, the diesel excise should be much higher than the excise on gasoline for two reasons: diesel is more polluting than gasoline and more importantly, through the better fuel efficiency, diesel cars contribute less fiscal revenues per mile. (C) 2011 Elsevier Ltd. All rights reserved.