This study shows diethyl phthalate (DEP) and dibutyl phthalate (DBP) were substantially adsorbed by activated sludge and its extracellular polymeric substance (EPS). The adsorption characteristics followed Freundlich and Langmuir isotherms. According to the Langmuir isotherm, each gram of activated sludge at maximum adsorbed 0.73 mg of DEP and 17.6 mg of DBP, and each gram of centrifugation-extracted EPS adsorbed 14.3 mg of DEP and 10.6 mg of DBP. The adsorption increased with the hydrophobicity of phthalates. This suggests most phthalates, which are of higher hydrophobicity than DEP and DBP, are likely to be removed from wastewater through adsorption by the activated sludge in the biological treatment process.