The subject of this study is to compare two different methods of converting heat to mechanical work. The first one is the well known Rankine cycle, which uses a pure substance as the working fluid. The second one is a power cycle which uses a mixture of substances with different boiling points as the working fluid. For convenience, this cycle will be called the mixed cycle hereafter. Since the conventional Rankine cycle has been analyzed by many other authors, we will focus on the mixed cycle. The main difference between the two is that, in thy mixed cycle, steam is released by heating a mixture. The initial working fluid is created as the expanded steam is absorbed by a mixture of lower concentration at lower temperature and pressure. The main disadvantages of the mixed cycle are the upper temperature limit and the larger mass circulation. In order to illustrate the ＇＇efficiency limits＇＇ of the mixed cycle, a theoretical analysis is presented, in which all processes are considered to be ideal.