The influence of the COD to sulphate ratio on the substrate consumption kinetic parameters was evaluated using a differential reactor filled with polyurethane foam matrices, taken from a horizontal-flow anaerobic immobilised biomass (HAIB) reactor. Experiments were carried out with synthetic substrate containing glucose, ammonium acetate and methanol, besides sodium bicarbonate. Micronutrients were provided by adding salts and trace metals from a stock solution. The influent chemical oxygen demand (COD) was maintained as constant as possible around 2 000 mg·l-1 in all the experiments. The reactors were subjected to increasing sulphate concentrations from 100 to 2 800 mg·l-1. First-order apparent kinetic parameters (K1 a pp ) decreased from 1.96 x 10-4 to 1.55 x 10-4l·mg VSS-1·h-1 as the COD to sulphate ratio decreased from 22.6 to 0.8. The progressive but moderate inhibition of the organic matter conversion observed indicates that high sulphate concentrations can be accommodated in HAIB reactors even at very low COD to sulphate ratios. WaterSA Vol.28(2) 2002: 213-216