In a method and an arrangement for checking the tightness of a vessel (10), especially of a tank-venting system of a motor vehicle, the vessel (10) is charged with a pressure generated by a motor-driven pressure source (30) and the motor current is determined. A reference leak (36) is selectively driven by a switchover valve (32) and is charged with a corresponding pressure and the motor current is determined. The determined electrical characteristic quantity and the determined reference characteristic quantity are compared and a conclusion is drawn therefrom as to the presence of a leak. To prevent the entry of moisture into the pressure source (30), it is provided that at least the pressure source (30) is heated intermittently. The heating takes place preferably by driving the switchover valve (32) at a suitable pulse duty factor. To prevent the entrance of moisture into the pressure source (30), a check valve (44) is provided and is suitably mounted in the connecting part of the pressure source (30).