A method for checking the tightness of a tank system of a vehicle wherein one introduces, with the aid of a pressure source, an overpressure relative to the atmospheric pressure over a pregiven time interval alternately into the tank system and a reference leak of a defined size connected in parallel to the tank system and detects at least one operating characteristic variable of the pressure source when introducing the pressure into the tank system (tank measurement) as well as when introducing into the reference leak (reference measurement) and compares the tank measurement to the reference measurement and, when there is a deviation of the tank measurement from the reference measurement by a pregiven value, drawing a conclusion as to a leak, is characterized in that, when a conclusion as to a leak is drawn, introducing pressure into the tank system over a further time interval, lengthening the tank measurement, thereafter executing a renewed reference measurement and outputting a fault announcement “leak” only when, by the latest, after the elapse of the further time interval, the tank measurement deviates from the renewed reference measurement likewise by a pregivable value.