Method for operating an arrangement for using waste heat of an internal combustion engine, wherein the internal combustion engine has an exhaust gas duct and the arrangement for using waste heat has a circuit conveying a working medium. In the circuit are arranged, in the flow direction of the working medium, a pump, at least one evaporator, an expansion machine and a condenser. The at least one evaporator is also arranged in the exhaust gas duct, wherein in the at least one evaporator an exhaust gas expelled from the internal combustion engine is used as a heat source, and thus the working medium is evaporated in the evaporator. The method according to the invention detects, inside the at least one evaporator, a leakage of the working medium into the exhaust gas duct.