A coolant system is disclosed, which is arranged to be driven by a prime mover of an aircraft. The coolant system comprises a fluid circuit (201) with a fluid therein, the fluid for cooling an electricity generator (208) located in the fluid circuit. The fluid circuit comprises: a cooling path (202) passing via at least one cooled component (220) of the generator; a first fluid flow source (221) configured to deliver a first fluid flow to the cooling path; a second fluid flow source (222) configured to generate a second fluid flow; and a valve arrangement (212) configured to selectively direct at least a proportion of the second fluid flow away from the cooling path in dependence on a measured operational parameter of the generator. A further aspect of the disclosure relates to an aircraft propulsion system and an aircraft comprising the coolant system.