An optical cell was used in this work, which allows spectroscopic measurements of fluids up to 600 degrees C and 300 MPa. The maximum pressure reached in this work was 254 MPa and the internal cell temperature was a maximum of 563 degrees C at a heating temperature of 617 degrees C. In the following, a combination of Raman and UV/vis spectroscopy was used for in situ investigation of the breakdown of sodium azide above 300 degrees C combined with the formation of sodium amide. It was verified that the breakdown of NaN3 occurs before the formation of NaNH2 starts. In a second step the decomposition of supercritical ammonia under ammonothermal conditions was investigated solely via Raman spectroscopy. The study comprises four different fluid compositions: pure ammonia, ammonia with a ruthenium catalyst, ammonia and mineralizer (sodium azide) and last ammonia, mineralizer and gallium nitride (GaN). By this, it was possible to proof that with pure ammonia and without a proper catalyst the decomposition of ammonia stops quite far before chemical equilibrium is reached.