Abstract The temperature dependence of the 81Br nuclear quadrupole resonance (NQR) frequencies and spin-lattice relaxation times (T1Q) of 81Br NQR have been measured in aluminum tribromide–ammonia (1/1), AlBr3·NH3, crystals. The 81Br NQR spectrum consists of 6 lines, which are ascribed to two AlBr3 groups. This has been confirmed by the observation of 27Al NQR using the 81Br ↔ 27Al spin echo double resonance method. A 81Br T1Q measurement revealed that the activation energies for the reorientation of the AlBr3 groups or the molecular tumbling of AlBr3·NH3 are 61.1 and 81.6 kJ mol−1. The difference in the activation energies was reflected in the temperatures of the fade-out of the 81Br NQR lines at Tf = 320 and 350 K, respectively.