The reaction pathways of disulfur (S-2) with oxygen (O-2) on the triplet and singlet potential energy surfaces have been investigated at the G3B3//B3LYP/6-311++G(3df,3pd) level. The calculated results reveal that P-2(trans-(OSSO)-O-1), P-3(cis-(OSSO)-O-1) and P-4(S-3 + (SO2)-S-1) produce only on the singlet energy potential surface while, P-1((SO)-S-3 + (SO)-S-3) produces on the both triplet and singlet surfaces. No intersystem crossing has been found between triplet and singlet surfaces and the energy barriers of the triplet surface are much lower than singlet surface. Therefore, S-2 + O-2 reaction proceeds only on triplet surface to produce P-1((SO)-S-3 + (SO)-S-3). (C) 2010 Elsevier B.V. All rights reserved.