Decolorization and inhibition kinetics of C.I. Direct Black 38 (DB 38) at concentrations varying between 200 and 3200mg l(-1) were investigated with partially granulated anaerobic mixed culture using glucose (3000 mg l(-1) COD) as carbon source and electron donor during batch incubation. Various models reported in literature (Monod, 0, 1 and 2 order) were tested in order to determine the most suitable substrate removal kinetic. The interpretation of kinetic data showed the first order kinetic was found to be the most appropriate kinetic among the kinetic models tested. Increases in dye concentrations from 0 to 3200 mg l(-1) reduce the degradation rate constant (k(1)) values from 0.0062 to 0.0017 h(-1) in batch studies performed with DB 38. Decolorization was achieved effectively under test conditions but ultimate mineralization of azo dyes was not observed. The course of the decolorization process approximates to second order kinetic at 200, 400, and 800 mg l(-1) of BD 38, and to first order kinetic at 1600 and 3200 mg l(-1) of BD 38 concentration. Aromatic amine accumulation was observed proportionally at a higher DB38 concentration. A competitive kinetic model that describes the anaerobic co-metabolism of increasing DB 38 dye concentrations with glucose as co-substrate has been developed based on the experimental data. The competitive inhibition constants were 1520 and 400 mg l(-1) for 200 and 1600 mg l(-1) of DB 38 dye, respectively. The average competitive inhibition constant was 914 +/- 574mg l(-1).The slope of inhibited reaction is K-S/R-max, (1 + I/K-ID). However, the R-max values did not significantly vary. The half saturation constants were found to be greater in high azo dye concentrations compared to the dye-free samples. The slope of inhibited reactor increased by a factor of (1 + I-D/K-ID). (C) 2003 Elsevier Inc. All rights reserved.