Torsional isomerization of tetrachlorohydroquinone in a low-temperature argon matrix has been investigated by Fourier transform infrared spectroscopy with the aid of density-functional-theory calculations. Infrared bands of the more stable isomer, trans, were observed in an argon matrix at 16 K. Those of the less stable isomer, cis, were observed during UV irradiation but disappeared immediately after the irradiation was discontinued. The isomerization from trans to cis occurred in the dark despite the high torsional barrier, 19.1 kJ mol(-1), when the matrix temperature was increased from 16 to 30 K. The enthalpy difference between the two isomers was estimated from the observed population changes to be 0.46 +/- 0.10 kJ mol(-1) (39 +/- 8 cm(-1)) by a least-squares fitting. This isomerization in low-temperature matrixes is ascribed to the matrix-induced hydrogen-atom tunneling.