An arrangement and a method for converting thermal energy to mechanical energy. The arrangement has a line circuit (3), circulation device (4) for circulating a zeotropic refrigerant mixture in the line circuit (3), an evaporator (6) in which the refrigerant mixture is vaporized by a heat source (7), a turbine (9) driven by the vaporized refrigerant mixture, and a condenser (12) which cools the refrigerant mixture so that it condenses. A control unit assesses whether the refrigerant mixture does not become fully vaporized in the evaporator (6) and, leads incompletely vaporized refrigerant mixture leaving the evaporator to a separating device (14) in which a liquid portion of the refrigerant mixture is separated from the gaseous portion, after which only the gaseous portion proceeds towards the turbine (9).