For over five decades, the high-temperature behaviour of ammonium dihydrogen phosphate, NH4H2PO4, has been a controversial subject; while initial works associate ionic conductivity increase around Tt=153°C to a physical transformation (structural phase transition), later research supports the chemical nature of the transformation. However, currently, the origin of the ionic conductivity increase is still not clear. To provide a possible interpretation for this phenomenon, a careful high-temperature thermal examination of this acid salt was conducted by means of Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Mass Spectroscopy (MS). The results show that when this acid salt is heated through Tt, a chemical decomposition into phosphoric acid (H3PO4) and ammonia (NH3) takes place. Considering that H3PO4 exhibits conductivity values around 10-3S·cm-1, our results suggest that the conductivity increase observed at around Tt is an exclusive consequence of the presence of this decomposition product.