The invention relates to an air conditioner, in particular for use in motor vehicles, with a cooling circuit and a ventilation system, wherein the cooling circuit features an evaporator/heat exchanger (12) that is arranged in the ventilation system, and wherein a gas sensor (30) is provided in the ventilation system downstream from the evaporator/heat exchanger (12) to detect the rates of a refrigerant of the evaporator/heat exchanger (12) of the air conditioner in the conveyed air-conditioned air. It is provided that the gas sensor (30) is arranged at a narrow point (32) and/or in an area of turbulent flow of an air guidance channel (10) of the ventilation system. The invention also relates to a method to operate such an air conditioner.