A compensation reservoir is provided that includes a connecting socket and a cover having a bottom and a circumferential collar for attachment to the connecting socket, preferably by thread means. The attachment is configured for air interchange between the air of the ambient atmosphere and an inner space of the reservoir which inner space is limited by the bottom. Air interchange is provided by an integrated duct that interconnects a radical outward section of the inner space and an opening within the cover, wherein lateral walls of the duct are casted in one piece from the material of the cover.