Transformations of di-n-butyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) have been investigated in anaerobic/anoxic/oxic (A/A/O) leachate treatment processes. Although the DBP removal processes are different when the DBP initial concentration is different, the overall system DBP removal efficiencies are high (>94%). DEHP is much more difficult to remove than DBP. The removal efficiency of DEHP is approximately 75%-78%. The results of mass balance calculations indicate that approximately 33.7%-50.7% of the DBP is degraded by the activated sludge, 48.9%-64.9% accumulates in the system, and 0.4%-1.4% is contained in the final effluent Approximately 15.0%-19.0% of the DEHP is degraded by activated microcosms, 75.8%-79.0% accumulates in the system, and 52%-6.0% is contained in the final effluent. Biodegradation and adsorption to the activated sludge are the main mechanisms for DBP removal and adsorption to the activated sludge is the main mechanism for DEHP removal. The different removal mechanisms of the two PAEs may be related to their different molecular structures. However, PAEs are not really removed when they adsorb onto the sludge. Therefore, methods for decreasing PAEs adsorption and increasing the biodegradation efficiencies of the leachate treatment processes should be further investigated. (C) 2019 The Chemical Industry and Engineering Society of China. and Chemical Industry Press Co., Ltd. All rights reserved.