The highly hygroscopic MoCl5 exhibited, under helium stream, several transformations between room temperature and 387 degrees C. At low temperatures (between 77 and 202 degrees C), the salt transforms into the stable MoO3 oxide and the less-stable oxochloride, MoOCl4, which evaporates and decomposes into MoOCl3. The former compound evaporates at 313 degrees C, while the entire vanishing of MoCl5 occurs at 387 degrees C. In the presence of NH4+-ZSM-5 zeolite, the starting salt is transformed into MoOCl4 and then into the volatile MoO2(OH)(2). The non-isothermal solid-state ion exchange of MoO2(OH)(2(g)) into H+-ZSM-5 zeolite occurs between 342 and 523 degrees C. Nonetheless, the isothermal exchange is highly recommended at a temperature below 523 degrees C in order to avoid the irreversible zeolite dealumination. In this study, thermodynamical calculations were used for the elucidation of the physical states of decomposition products.