Abstract: In order to control the temperature of the transmission oil of a motor vehicle driven by a liquid-cooled engine, a coolant/transmission oil heat exchanger is provided. The coolant stream supplied to the heat exchanger is miscible in a thermostatic valve from recooled and non-recooled coolant in order to achieve a desired temperature. Preferably the thermostatic valve is controlled by the temperature of the transmission oil, but it is also possible to have two expanding material elements as control pistons of the thermostatic valve, with one control piston being provided in the oil chamber of the thermostatic valve and the other control piston being provided in the coolant mixing chamber of the thermostatic valve. It is also possible, for purposes of precontrol, to electrically heat at least one of the control pistons.