Theoretical study has been carried out on the fate of methylthiomethylperoxy radical (CH3SCH2OO, MSP) in the atmosphere. The intramolecular H-shift followed by recombination with O-2, MSP -> CH2SCH2OOH -> OOCH2SCH2OOH (MSPO2), is found to be fast enough, that is, 2.1 s(-1) at 293 K, to compete with and even surpass the possible bimolecular reactions of MSP with NOx, HO2, and RO2 in the remote marine atmosphere. MSPO2 would also undergo another intramolecular H-shift and decompose to the most important intermediate HOOCH2SCHO instead of the CH3SCH2O radical. HOOCH2SCHO would be further oxidized via the route as HOOCH2SCO (by OH radical) -> HOOCH2S (by decomposition) -> HOOCH2SO (by O-3 or NO2) -> HOOCH2SO2 (by O-3 and NO2) -> OH + CH2O + SO2 (by decomposition). Our calculations suggest a drastically different oxidation mechanism for dimethyl sulfide (CH3SCH3, DMS) in the remote marine atmosphere.