Abstract: The invention relates to a split cooling circuit (16) of an internal combustion engine (17), with a cylinder head water jacket (18) and an engine block water jacket (19) being provided. The split cooling circuit (16) has a pump (21), a radiator (22), a thermostat (23) and a heater core (24), with a coolant circulating in the split cooling circuit (16). To considerably reduce a warm-up phase of the internal combustion engine, the thermostat (23) is arranged to simultaneously control a flow of the coolant through the engine block water jacket (19) and through the radiator (22) when the coolant exceeds a predetermined temperature.