Abstract: A treatment device for treating exhaust gases of a fuel cell stack of a fuel cell system is provided. A humidifier, in which the exhaust gas is usable for humidifying a medium capable of being introduced into the fuel cell stack, includes a water separator for reducing a content of liquid water in the exhaust gas. The humidifier and the water separator are integrated in a common constructional unit. An outflow area of the humidifier has a cross-section capable of being passed, which is larger than a cross-section of an inflow area of the humidifier capable of being passed. The constructional unit includes at least one collection area for the liquid water separated upon exit from the outflow area.