In　an　attempt　to　assist　the　experimental　findings,　density　functional　theory　(DFT)　studies　at　the　B3LYP/6-311+G(3df,2p)　level　on　the　structures,　vibrational　frequencies　and　relative　stabilities　of　disulfides　XSSY　and　their　isomeric　thiosulfoxide　structures　SSXY　where　X　and　Y　are　H,　F,　Cl,　Br　and　I,　have　been　reported　in　the　present　paper.　The　agreement　with　the　available　observed　values　experimentally　is　good.　The　calculated　results　suggest　that　the　linear　structures　are　more　stable　than　their　branched　structures　thermodynamically　for　all　S2XY　isomers.　Thiosulfoxide　difluoride　SSF2,　which　is　the　only　disulfide　known　to　exist　in　two　stable　isomeric　forms,　is　the　most　closest　to　its　disulfide　isomer　energetically　in　all　tautomers　discussed　in　this　paper.　The　activation　energies　for　the　unimolecular　isomerization　at　room　temperature　were　also　predicted　at　the　same　level,　and　the　calculated　results　indicated　that　the　chain　forms　of　XSSY　are　stable　with　respect　to　their　corresponding　branched　forms　kinetically.　The　intramolecular　atom　transfer　rearrangement　reaction　are　not　likely　processes　because　of　the　high　potential　barriers.