Torsional isomerization around the two C-O bonds of 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone in a low-temperature argon matrix has been investigated by FTIR spectroscopy with an aid of the density functional theory calculations. The infrared spectrum of the most stable isomer, which has two intramolecular hydrogen bonds of C=(OH)-H-...-O, was observed, while that of a less stable isomer, which has C=(OH)-H-...-O and C-(ClH)-H-...-O hydrogen bonds, was observed as a transient species during UV-visible irradiation. Isomerization from the less stable isomer to the most stable one occurred immediately after the irradiation was stopped. This isomerization is ascribed to the effect of hydrogen-atom tunneling.