A retaining device for frictionally retaining two mobile parts of a thermostatically controlled mixing valve. The first part (2) of the mixing valve contains a cavity having a side wall (40), a top wall (41) and a bottom wall (42). An O-ring type packing washer (9) is disposed in the cavity and frictionally grips a surface of the second mobile part (13). The axial distance between top wall (41) and bottom wall (42) of the cavity is less than the cross-sectional diameter in the axial direction of the ring-shaped packing washer (9). The packing washer (9) is thus compressed axially and establishes a frictional hold between the top wall (41) and bottom wall (42) of the cavity and the axially extending surface of the second mobile part (13). The diameter of the cavity is greater than the diameter of the compressed ring-shaped packing washer (9) thereby forming a ring-shaped space or chamber between the washer (9) and the side wall (40). This ring-shaped space communicates with a water holding area of the mixing valve through a groove (43) in the bottom wall (42) of the cavity.