This　study　investigated　biodegradation,　transformation　and　sludge/water　partition　of　two　natural　estrogens　-　estrone　(E1)　and　17　beta-estradiol　(E2)　-　in　a　pilot-scale　step-feed　anoxic/oxic　(A/O)　wastewater　treatment　system.　The　results　show　that　the　total　amounts　of　E1　and　E2　(E1　+　E2)　in　the　water　phase　were　approximately　9-23%　of　those　in　the　sludge　phase　in　the　reactor　tanks.　The　estrogens　accumulated　in　the　return　sludge　led　to　the　increase　in　the　estrogen　concentration　in　the　first-stage　anoxic　zone.　The　biodegradation　of　E1　and　E2　in　the　first-stage　aerobic　zone　was　highly　efficient.　The　sludge/water　partition　coefficients　(Log　K-d)　of　E1　and　E2　were　gradually　increased　from　2.3　and　2.6　in　the　first　stage　to　3.2　and　3.3　in　the　third　stage,　respectively,　and　Log　K-d　of　E2　was　higher　than　that　of　E1　in　each　A/O　stage.　In　addition,　the　Log　K-d　values　of　E1　and　E2　were　larger　in　the　aerobic　zones　than　in　the　anoxic　zones.　Under　six　operation　conditions　tested,　there　was　a　linear　relationship　between　the　concentrations　of　E1　and　E2　in　the　water　phase,　indicating　the　transformation　between　E1　and　E2.　The　calculated　mutual　conversion　equilibrium　constant　(K)　for　the　transformation　between　E1　and　E2　was　significantly　higher　in　the　anoxic　zones　(0.38-0.81)　than　in　the　aerobic　zones　(0.08-0.24).　(C)　2013　Elsevier　B.V.　All　rights　reserved.