The kinetics of aerobic biological oxidation of a PTA (terephthalic acid) plant waste was investigated for plant effluent (initial COD: 7000-8000 ppm) and anaerobically treated plant effluent (initial COD: 1500-2500 ppm). Experiments were performed at three different Dissolved oxygen (DO) levels viz. low (0.05-0.1 ppm), intermediate (1.00-3.00 ppm) and high (> 6.00 ppm). For each DO level at least three mixed liquor suspended solids (MLSS) loadings were studied in the range of 2000-10 000 ppm. Care was taken to eliminate mass transfer resistance for oxygen transfer at gas-liquid and liquid-solid interfaces. An attempt has been made to propose the kinetics of reaction. In order to confirm the rate equation, experiments were also performed in continuous mode of operation over a wide range of MLSS and dissolved oxygen level. A comparison between batch and continuous mode results has been presented.