A locking mechanism for a device (1) to modify the control timing of cylinder valves of an internal combustion engine, in particular for a vane-cell positioning device. The device (1) consists of a drive wheel (2) driven by a crankshaft of the internal combustion engine, said drive wheel having a cavity (6), and an impeller (8) permanently connected to the camshaft (7) that has at least one vane (12). In the cavity (6) of the drive wheel (2), at least one working chamber (15) is formed by intermediate walls (14) and each of these chambers is divided by an associated vane (12) into two hydraulic pressure spaces (16, 17). When pressure is applied by means of a hydraulic pressure medium, the pressure spaces (16, 17) effect a pivoting motion of the impeller (8) with respect to the drive wheel (2), whereas when the pressure is not applied to one of the pressure spaces (16, 17) the impeller (8) and the drive wheel (2) are mechanically coupled together. The mechanical coupling between the impeller (8) and the drive wheel (2) of the device (1) is accomplished by at least one axially moving vane (12) of the impeller (8) which is designed both as a vane pivoting element and as a locking element.