Gas-phase reaction of dimethyl sulfoxide (DMSO) with OH radical is investigated using ab initio, calculations. The CH3S(O)OH + CH3 product channel has an overall negative reaction activation energy, and it could proceed by formation of an addition complex (CH3)(2)S(O) . OH and subsequent dissociation into CH3S(O)OH + CH3 via an energy barrier at -29.8 kJ/mol below DMSO + OH. The other two product channels, CH3S(O)CH2 + H2O and CH3SO+CH3OH, have energy barriers of 12.4 and 78.4 kJ/mol above DMSO+OH, respectively. The CH3S(O)OH+CH3 product channel is likely the dominant pathway in the DMSO + OH reaction. (C) 2002 Published by Elsevier Science B.V.