Abstract In the scenario of increasing of the worldwide energy demand an important role can be played by the Organic Rankine cycle (ORC). Due to the nature of the working fluids, they partially decompose with the temperature. The main focus of this paper is to evaluate the behavior of the power plants that operate with a partially decomposed organic fluid. The investigated fluids are MDM and isopentane. An explorative investigation of the plant with pure fluid is performed in order to evaluate the general behavior of the plant varying the conditions of the heat source and of the heat rejection. Moreover, off-design analysis of the cycles when the working fluid encounters thermal decomposition is proposed. Off-design analysis shows that properties of decomposed products could significantly affect cycle efficiency. Both fluids decomposes with formation of methane. The presence of this one has high impact on the condensation and therefore on the decreasing of performances of the power plant.