Abstract: The present invention relates to a heat exchanger for cooling a gas which can mainly be applied in EGR systems with a floating core. The differences in temperature achieved during operation of the casing and the core of gas ducts housed therein give rise to degrees of expansion which are also different. If the ends of both components were fixed to each other, stresses which would cause the breakage thereof would occur. The common solution applied is to leave one of the ends of the core of ducts floating, i.e., with capacity for longitudinal displacement with respect to the casing to prevent the occurrence of stresses. The floating end of the core has an attachment by means of O-ring gaskets. The O-ring gaskets are made of an elastomer that cannot reach very high temperatures, hence in the state of the art the floating attachment is on the side where the already cooled gas exits.