The invention relates to a method for operating an air conditioning system for a vehicle including the steps of:calculating a water collection quantity in an evaporator during the operating period of a cooling mode of the air conditioning system,calculating a quantity of drainage water from the evaporator,determining a quantity of water present in the evaporator at a certain point in time, andcomparing the determined quantity of water in the evaporator with a predetermined limit value at the point in time of starting the vehicle or at the point in time of starting the ventilation mode of the air conditioning system.The cooling mode of the air conditioning system is activated if the limit value is exceeded and otherwise the ventilation mode of the air conditioning system is activated.