An exhaust gas heat exchanger comprising an external shell (20) extending between two tube plates (30, 31) and defining a coolant chamber; internal tubes (38) forming exhaust gas passages which extend between the tube plates (30, 31); an exhaust gas manifold divided by a baffle plate (29) into first and second chambers (26, 28) with an exhaust gas inlet (25) and outlet (27) respectively. The baffle plate (29) is provided with a valve (51) which can be operated between an open position, which exhaust gas flows along the cooling tubes (38), and a closed position, in which exhaust gas is diverted directly from the first chamber (26) to the second chamber (28).