Measurements of spin-lattice relaxation time T₁ have been performed for polycrystalline ammonium bromide as a function of hydrostatic pressure and temperature. Activation parameters for NH₄⁺ ion reorientation as functions of pressure were obtained. A two-well potential model is proposed for description of ammonium reorientation motion. On the basis of the proposed model and obtained activation data the libration frequency of the ammonium ion as a function of pressure was calculated and good agreement between experiment and calculation was obtained.