The matrix isolation technique, combined with infrared spectroscopy and theoretical calculations, has been employed to investigate the thermal and photochemical reactions of dimethylsulfoxide (DMSO) with CrCl 2 O 2 and OVCl 3 . Twin jet codeposition leads to formation and isolation of a photochemically unstable 1:1 complex. The photoproduct in the twin jet DMSO+CrCl 2 O 2 experiments is identified as dimethyl sulfone, (CH 3 ) 2 SO 2 , interacting with the Cl 2 CrO fragment, while in the analogous OVCl 3 reaction, the photoproduct bands were too weak to allow product identification. Merged jet codeposition led to rapid gas phase reaction, and in the case of DMSO+CrCl 2 O 2 , dimethyl sulfone is formed in high yield. This marks the first demonstration of a gas phase thermal oxygen atom transfer from CrCl 2 O 2 to an organic substrate. For the reaction of DMSO with OVCl 3 , no volatile products are deposited in the matrix and dimethyl sulfone is not a product. These results support differing pathways for the reactions of CrCl 2 O 2 and OVCl 3 , a conclusion that is supported by density functional calculations.