Very recently, we have interestingly found that the Fe(II)-activated peroxydisulfate (PDS) process can oxidize methyl phenyl sulfoxide (PMSO) to its oxygen transfer product methyl phenyl sulfone (PMSO2), suggesting the possible occurrence of a ferryl intermediate (Fe(IV)) rather than long recognized sulfate radical (SO4 center dot-) in this process. In this work, the formation of PMSO2 was also observed when PMSO was treated by the Fe(II)/peroxymonosulfate (PMS) system over the pH range of 3-7, and the yield of PMSO2 (i.e., the molar ratio of PMSO2 formed to PMSO lost) was quantified to be approximately 100% under acidic conditions, similar to the Fe(II)/PDS system. Moreover, O-18 from isotope-labeled water ((H2O)-O-18) was successfully incorporated into PMSO2 product when PMSO was oxidized by the Fe(II)/PDS and Fe(II)/PMS systems in (H2O)-O-18, likely due to the competitive oxygen exchange of Fe(IV)-oxo species with (H2O)-O-18. Further, Fe(IV) evolution profiles in both systems were predicted by a Fe(IV)-mediated kinetic model, and these results are in good agreement with the experimental data obtained with a stopped-flow spectrophotometer.