An　endocrine　disrupting　chemical,　di-(2-ethylhexyl)　phthalate　(DEHP),　can　be　removed　from　wastewater　by　biological　aerated　filters,　but　the　removal　mechanism　is　not　clear.　This　original　study　focused　on　the　adsorption　isotherm　and　kinetics　of　DEHP　on　biofilms　from　a　biological　aerated　filter　at　4　degrees　C.　This　low　temperature　was　selected　to　reduce　interactions　between　the　biological　metabolism　and　adsorption　as　much　as　possible.　The　experimental　data　at　4　degrees　C　fit　with　the　Langmuir　and　Freundlich　isotherm　models.　R-2　for　the　Langmuir　and　Freundlich　isotherm　is　0.969　and　0.949,　respectively.　For　the　Langmuir　isotherm,　q(max)　(Langmuir　adsorption　capacity)　is　161.55g/g　and　k(L)　(Langmuir　constant)　is　0.186L/g.　For　the　Freundlich　isotherm,　n　(Freundlich　constant)　is　1.68　and　k　(Freundlich　adsorption　capacity)　is　29.46g/g.　The　adsorption　capacity　of　1g　biofilm　for　DEHP　is　34.67g　as　calculated　from　pseudo-second-order　kinetics,　which　corresponds　well　with　adsorption　rate　data.　There　was　a　dynamic　balance　of　DEHP　concentration　between　biofilms　and　wastewater.　Without　biological　activity,　DEHP　would　migrate　from　the　biofilms　back　into　the　wastewater　if　the　concentration　of　DEHP　in　biofilms　becomes　higher　than　that　in　the　wastewater.　These　results　are　fundamental　to　understand　DEHP　removal　mechanisms　in　a　biological　aerated　filter.