A method for determining an air mass in an air separator of a water direct injection system for injecting a water/fuel mixture into a combustion chamber of an engine of a motor vehicle. The air separator is disposed between a water pump for delivering water of the water/fuel mixture and a high-pressure pump for feeding the water/fuel mixture to a high-pressure injector for injecting the water/fuel mixture into the combustion chamber. The method includes increasing a pressure of the water from a first pressure value to a second pressure value by the water pump, determining a water volume delivered by the water pump during the increasing of the pressure of the water by the water pump, and determining the air mass in the air separator on a basis of the determined water volume delivered by the water pump.