Iodinated　aromatic　disinfection　byproducts　have　attracted　much　attention　owing　to　their　high　toxicity.　However,　little　has　been　known　about　the　iodination　mechanism　to　date.　In　this　study,　the　iodination　of　model　aromatic　precursors,　tyrosine　(Tyr)　and　its　model　dipeptides,　by　HOI　and　other　iodinating　agents　was　explored　using　a　quantum　chemical　computational　method,　and　the　halogenation　of　Tyr　by　HOX　(X　=　Cl,　Br,　and　I)　was　compared.　The　results　indicate　that　the　phenolate　salt　plays　a　key　role　in　the　iodination　of　the　phenol　ring　in　Tyr　compounds　by　HOI　via　the　classic　SEAr　mechanism　and　the　second　deprotonation　becomes　the　rate-limiting　step,　which　explains　why　the　kinetic　isotope　effect　(KIE)　was　observed　in　the　iodination　of　aromatic　compounds.　Among　the　seven　possible　iodinating　agents　present　in　chloramination,　HOI　is　the　predominant　one,　and　I2　is　the　second　in　the　iodination　of　the　phenol　ring　under　typical　chloramination　conditions.　In　the　further　investigation　of　bromination　and　chlorination,　the　KIE　was　found　to　also　occur　in　the　bromination　of　Tyr.　More　importantly,　the　different　reactivity　orders　of　HOX　reacting　with　the　phenol　ring　and　the　amino　group　in　Tyr　are　related　to　the　hardness　of　both　HOX　and　substrates,　which　can　be　evaluated　from　the　energy　gap　(ELUMO-HOMO)　between　the　LUMO　and　HOMO　energies.　Following　the　＂like-attracts-like＂　principle,　the　halogenating　agent　prefers　the　substrate　with　a　similar　ELUMO-HOMO　value.　The　results　are　helpful　in　further　understanding　the　iodination　mechanism　and　identifying　various　halogenated　products.　The　iodination　mechanism　of　Tyr　compounds　by　HOI　and　other　iodinating　agents　was　studied　using　the　DFT　method,　and　the　halogenation　of　Tyr　by　HOX　(X　=　Cl,　Br,　and　I)　was　compared.