Biotreatment of wastewater from a dimethyl terephthalate (DMT) plant in repeated batch mode with 24-h cycles and feed chemical oxygen demand (COD) of 2000 mg/ l, led to a COD reduction of approximately 86% in 20 h. Incorporation of porous plastic biomass support particles (BSP) in the reactor however, resulted in similar COD reduction in only 6 h. The increased percentage COD reduction in BSP reactor was found to be related to the increase in biomass (from 2.8 to 10.5 g/ l) in the reactor. In the continuous mode, the optimum organic loading and hydraulic retention time (HRT) for more than 90% COD reduction were found to be approximately 14 kg COD/m 3/d and 12 h, respectively, with the BSP reactor. On the other hand, only 40–45% COD reduction was observed under the same conditions in the reactor not containing the support particles. Reduction in percentage COD reduction at high organic loading rates was due to the presence of formaldehyde in the wastewater feed. The control reactor, not containing BSP, was found to be sensitive to formaldehyde concentrations above 160 mg/ l, while the reactor with BSP was resistant up to a formaldehyde concentration of 410 mg/ l. The advantages of the BSP augmentation for the aerobic biotreatment of DMT wastewater are discussed.