The　relationship　between　nonylphenol　(NP)　isomer　structure　and　its　biodegradability　within　the　waste-water　treatment　process　of　sequencing　batch　reactor　(SBR)　was　investigated.　The　GC-MS　method　was　used　for　detecting　the　NP　isomers　existing　in　the　SBR　influent,　activated　sludge　and　effluent.　Fifteen　NP　isomers　were　detected　in　the　influent,　with　significant　biodegradability　variations　being　observed　among　these　isomers.　It　was　found　that　the　NP　isomers　associated　with　retention　time　of　10.553,　10.646,　10.774,　and　10.906　min　in　the　GC-MS　analysis　showed　higher　biodegradability,　while　the　isomers　with　retention　time　of　10.475,　10.800,　and　10.857　min　illustrated　lower　biodegradability.　Through　analyzing　the　mass　spectrograms,　the　chemical　structures　of　four　selected　NP　isomers　in　the　wastewater　were　further　deduced.　The　higher　correlation　coefficients　of　0.9421　and　0.9085　were　observed　between　the　NP　isomer　biodegradation　rates　and　the　molecular　connectivity　indexes　with　the　order　of　two　and　four,　respectively.　Such　correlation　analysis　indicated　that　a　more　complex　side　branch　structure　(such　as　a　larger　side　carbon-chain　branch　or　more　branches　in　the　nonyl)　of　NP　isomer　would　lead　to　lower　biodegradability,　and　a　longer　nonyl　chain　of　the　isomer　would　result　in　a　higher　biodegradability.　(C)　2009　Elsevier　Ltd.　All　rights　reserved.