A heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system of a vehicle, the heat exchanger system comprising a heat exchanger (1, 1b), wherein the heat exchanger (1, 1b) comprises: - a first guiding passage, - a second guiding passage, and - a flow reversing region arranged between the first guiding passage and the second guiding passage, whereby the first guiding passage is arranged to guide the flow of exhaust gases along a first flow direction towards the flow reversing region, and the second guiding passage is arranged to guide the flow of exhaust gases from the flow reversing region in a second flow direction, such that heat may be exchanged between the flow of exhaust gases in the second guiding passage and the flow of exhaust gases in the first guiding passage, - a nitrogen monoxide oxidation site for oxidising nitrogen monoxide to nitrogen dioxide (NO 2) in the flow of exhaust gases, the NO oxidation site comprises an NO oxidation catalyst, and - a further oxidation site for oxidising a further constituent of the flow of exhaust gases, the further oxidation site comprises a catalyst for oxidising the further constituent, characterised in that - the further oxidation site is arranged within the first guiding passage; - the NO oxidation site is positioned within the second guiding passage, downstream of the further oxidation site; - the NO oxidation site is positioned such that the flow of exhaust gases at a downstream end of the NO oxidation site in use of the heat exchanger system is arranged to proceed at a temperature within a predetermined temperature interval corresponding to a desired NO to NO 2 (NO:NO 2) ratio interval in the flow of exhaust gases, and - the further oxidation site is a hydrocarbon oxidation site for oxidising methane to mainly carbon dioxide (CO 2). - a first guiding passage, - a second guiding passage, and - a flow reversing region arranged between the first guiding passage and the second guiding passage, whereby the first guiding passage is arranged to guide the flow of exhaust gases along a first flow direction towards the flow reversing region, and the second guiding passage is arranged to guide the flow of exhaust gases from the flow reversing region in a second flow direction, such that heat may be exchanged between the flow of exhaust gases in the second guiding passage and the flow of exhaust gases in the first guiding passage, - a nitrogen monoxide oxidation site for oxidising nitrogen monoxide to nitrogen dioxide (NO 2) in the flow of exhaust gases, the NO oxidation site comprises an NO oxidation catalyst, and - a further oxidation site for oxidising a further constituent of the flow of exhaust gases, the further oxidation site comprises a catalyst for oxidising the further constituent, - the further oxidation site is arranged within the first guiding passage; - the NO oxidation site is positioned within the second guiding passage, downstream of the further oxidation site; - the NO oxidation site is positioned such that the flow of exhaust gases at a downstream end of the NO oxidation site in use of the heat exchanger system is arranged to proceed at a temperature within a predetermined temperature interval corresponding to a desired NO to NO 2 (NO:NO 2) ratio interval in the flow of exhaust gases, and - the further oxidation site is a hydrocarbon oxidation site for oxidising methane to mainly carbon dioxide (CO 2).