The present paper presents a thorough experimental investigation of mechanistic pathways of thermal decomposition of ammonia borane (AB) and its mixture with KBr. A comparative detection and temperature-dependent in situ monitoring of the decomposition products was done by use of temperature-dependent infrared (IR) spectroscopy of both solid (in transmission through KBr pellets and ATR mode) and gaseous products, thermogravimetry (TG) and evolved gas analysis mass spectroscopy (EGA-MS). This enables discrimination of the processes occurring in the bulk from those in the near-surface level. For the first time, a high influence of the KBr matrix on AB decomposition was found and thoroughly investigated. Although KBr does not change the chemical and physical identity of AB at ambient conditions, it dramatically affects its thermal decomposition pathway. It is found that the presence of KBr not only favors the production of diammoniate diborane in the induction phase, but also enables an efficient catalysis of AB decomposition by NH4+ ions, present at the KBr AB interface, which leads to suppression of emission of unwanted gaseous side products other than NH3. IR spectroscopy was also used to shed a light on the molecular background of the frequently observed, but never investigated increase of the mass of decomposition products.