Abstract: An arrangement and a method for cooling coolant in a cooling system in a vehicle: The cooling system includes a first cooling circuit, a first coolant pump (11) to circulate coolant through the first cooling circuit, a first radiator (13) to cool the coolant, and a thermostat (12) which opens to direct coolant to the first radiator (13) when the coolant is above the thermostat's regulating temperature (t2); and a second cooling circuit includes a second radiator (20) to cool coolant. A flow device (23, 29) selectively transfers a portion of coolant from the first cooling circuit to the second cooling circuit, so that this portion of the coolant is cooled in the second radiator (20) when the coolant in the first cooling circuit is within a temperature range defined by a lowest temperature (t1) at which the coolant initially needs cooling and a highest temperature which is equal to the thermostat's regulating temperature (t2).