Thermal stability and fluid compatibility with materials as well as with lubricants in contact is a critical step in the design of an ORC plant. The working fluid must have a high thermal stability to provide the desired lifetime and a cost-effective plant. Care should be taken to make sure that the combination fluid/lubricant/material can assure a long lifetime period of the plant. Among the materials used are copper, steel and stainless steel just to name a few. The lubricant can be miscible or immiscible with cycle fluid, but for minimum system complexity miscible oil is desirable. Chemical decomposition of the fluid not only reduces the plant efficiency and makes the replacement of the fluid necessary but can produce non­condensable gases which have corrosive effects on the materials of the system. In order to study the fluids decomposition, two methods are available: the dynamic loop tests and the static capsule tests. The first method is the best but time-consuming and expensive. In general, to safeguard against plant damage from fluid decomposition, a combination of static capsule tests, large safety margins and field monitoring is usually employed. Wali [8], Calderazzi and Colonna di Paliano [9], Angelino and Invernizzi [10] are some authors who investigated the thermal stability of fluids. In Table 3 are displayed the maximum stability temperature of some fluids.