Abstract. Phengite is known to be an important mineral in the transport of alkalis and water to upper mantle depths. Since ammonium (NH4+) can substitute for K+ in K-bearing minerals, phengite is thus a potential host to transport nitrogen into the mantle. However, the temperature and pressure conditions at which devolatilisation of NH4-bearing phengite occurs are not well constrained. In this study, NH4-phengite (NH4)(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2 was synthesised in piston-cylinder experiments at 700 ∘C and 4.0 GPa. Its devolatilisation behaviour was studied by means of in situ micro-FTIR (Fourier transform infrared) spectroscopy under low and high temperatures from −180 up to 600 ∘C at ambient pressure using a Linkam cooling–heating stage and pressures up to 42 GPa at ambient temperature in diamond anvil cell (DAC) experiments. In addition to these short-term in situ experiments, we performed quenched experiments where the samples were annealed for 24 h at certain temperatures and analysed at room conditions by micro-FTIR spectroscopy. Our results can be summarised as follows: (1) an order–disorder process of the NH4+ molecule takes place with temperature variation at ambient pressure; (2) NH4+ is still retained in the phengite structure up to 600 ∘C, and the expansion of the NH4+ molecule with heating is reversible for short-term experiments; (3) kinetic effects partly control the destabilisation of NH4+ in phengite; (4) ammonium loss occurs at temperatures near dehydration; (5) NH4+ in phengite is apparently distorted above 8.6 GPa at ambient temperature; and (6) the local symmetry of the NH4+ molecule is lowered/descended/reduced by increasing pressure (P) or decreasing temperature (T), and the type and mechanism of this lowered symmetry is different in both cases. The current study confirms the wide stability range of phengite and its volatiles and thus has important implications for the recycling of nitrogen and hydrogen into the deep Earth. Moreover, it is considered as a first step in the crystallographic determination of the orientation of the NH4+ molecule in the phengite structure.