The stability of 5-chloro-2-(2',4'-dichlorophenoxy)-phenol, triclosan (TCS) in solid state has been studied by nuclear quadrupole resonance (NQR) spectroscopy. The NQR spectrum of TCS at 77 K reveals three resonance lines of the frequencies 35.07, 35.534 and 36.189 MHz assigned to chlorine atoms at positions 5, 4', and 2' of the biphenyl ring. TCS in solid state is thermally stable, no temperature-induced release of chlorine or decomposition of this compound have been detected, which is consistent with the DTA results. The temperature dependence of the resonance frequency suggested the occurrence of small-angle torsional vibrations of the mean activation energy of 1.62 kJ/ mol. B3LYP/6-311++G** method reproduced very well the NQR resonance frequencies and the frequencies of the small-angle torsional vibrations for TCS. (C) 2008 Elsevier B.V. All rights reserved.