Interest　in　phlorotannins　has　increased　in　recent　years　largely　due　to　antioxidant　capacity,　however,　the　redox　mechanism　of　phlorotannins　is　still　obscure.　In　the　present　study,　the　electrochemical　oxidation　mechanisms　of　eckol　(EL)　and　phlorofucofuroeckol-A　(PFF-A),　two　representative　phlorotannin　compounds,　were　comparatively　analyzed　in　a　wide　pH　range　using　cyclic　and　differential　pulse　voltammetry　as　well　as　spectroscopic　assay.　The　voltammetric　study　revealed　that　EL　and　PFF-A　were　successively　oxidized　in　three　pH-dependent　steps.　Moreover,　it　was　found　that　the　PFF-A　presented　a　stronger　proton　and　electron　transferring　activity　as　compared　to　EL　since　PFF-A　exhibited　lower　acid–base　dissociation　constant　(pKa)　value　and　higher　heterogeneous　rate　constant　(kbh)　value　in　the　first　oxidation　step.　These　property　were　further　evidenced　by　comparison　of　direct　antioxidant　activity　(i.e.,　superoxide　anion　and　peroxide　radicals)　as　well　as　indirect　antioxidant　activity　(i.e.,　mRNA　expression　of　two　phase　II　enzymes)　between　EL　and　PFF-A.　Practical　applications:　Phlorotannins　from　edible　algae　have　been　regarded　as　novel　antioxidants　those　presented　high　application　potential　in　food　industry.　Even　though　antioxidant　activity　of　phlorotannin　compounds　have　been　widely　investigated　in　both　in　vitro　and　in　vivo　studies,　very　few　reports　focused　on　electron　transferring　functionality　which　is　chemical　basis　for　antioxidant　process.　Herein,　the　oxidative　mechanisms　of　two　representative　phlorotannins　were　comparatively　analyzed　using　multiple　electrochemical　methods.　This　is　hopefully　to　give　information　on　the　chemical　meaning　behind　the　antioxidant　activity　of　dietary　phlorotannins.　©　2019　Wiley　Periodicals,　Inc.