Torsional isomerization between cis and trans hydroquinone in low-temperature matrices has been studied by FTIR spectroscopy with an aid of DFT calculations, When the temperature of Xe matrix was changed between 16 and 75 K, the cis/trans population ratio changed reversibly in conformance with the Boltzmann distribution law, despite the high torsional barrier, 10.8 kJ mol(-1). The cis-trans enthalpy difference was estimated from the population changes to be 0.19 +/- 0.06 kJ mol(-1). The cis-trans isomerization equilibrium at low temperature is accounted for in terms of the matrix-induced tunneling of hydrogen atoms. (C) 2002 Elsevier Science B.V. All rights reserved.