The dynamic behaviour of the ammonium ion in synthetic ND4/NH4-phlogopite has been studied as a function of temperature by infrared spectroscopy, from room temperature to 20 K. Ammonium occupies the interlayer site in phlogopite. Orientational order-disorder of NH4+ is a well-established phenomenon in ammonium chloride (NH4Cl) and other ammonium halides and ammonium salts. Although ammonium chloride has higher symmetry than ND4/NH4-phlogopite, we anticipate similar dynamic behaviour of ammonium in the interlayer site of the phlogopite structure. Infrared spectra show noticeable changes on cooling. Using the autocorrelation analysis method we find distinct changes in the line width of the Gaussian profile fitted to the central autocorrelation peak. This is attributed to a transition of the ND4+ group, which is orientationally disordered at higher temperatures and becomes relatively ordered at temperatures below the transition temperature of around 130 K. The order parameter for the transition appears to follow classical second-order behaviour as a function of temperature.