Pre-combustion microbial desulphurization of coal containing total sulphur (3.90%) and pyritic sulphur (2.80%) has been evaluated in a coal slurry reactor. The coal slurry reactor operated at hydraulic retention time (HRT) of 96 h with a coal pulp density of 15 percent and remove 79 percent of pyritic sulphur and 76 percent of ash with an increase in the calorific value of coal from 4400 to 6800 kcal kg(-1) at a pyritic load of 1.9 kg pyritic sulphur kg(-1) MLSS d(-1). The treated coal yield is 72 percent. The biochemical kinetic coefficients, viz. yield coefficient (Y) and decay coefficient (K-d) in the coal slurry reactor system are 0.178 and 0.007 d(-1), respectively, while maximum growth rate (mu(max)) and half saturation rate constant (K-s) are 0.025 h(-1) and 0.220 g l(-1) as pyrite, respectively. (C) 2004 Elsevier Ltd. All rights reserved.