The elusive hydrogen-bonded sulfur monoxide-hydroxyl radical complex (circle OH center dot center dot center dot OS), a missing intermediate in the atmospheric chemistry of SO2, was generated in the 266 nm laser photolysis of the sulfinyl radical HOSO circle in cryogenic Ar-matrixes. In addition to the IR spectroscopic characterization with deuteration, its thermal conversion to HOSO circle with an activation barrier of 0.33 +/- 0.11 kcal mol(-1) (calcd 0.32 kcal mol(-1), CCSD(T)-F12a/AVTZ) in the temperature range of 15.0-21.0 K and a H/D kinetic isotope effect of 2.4 at 16.0 K have been observed.