The invention relates to a connecting piece (6) for fluid lines of an exhaust gas aftertreatment system (1), comprising a hollow body (6a) having at least three fluid connection points (14, 15), which are fluidically connected to each other in the hollow body (6a), at least two of the fluid connection points (15) being separated from a third fluid connection point (14) by means of respective membranes (19), each membrane (19) being permeable to a liquid and, when wetted by the liquid, being impermeable to a gas.