Batch studies have been conducted to examine the effect of various factors (which are suspected to limit the coal biodesulphurization process rate) on bacterial oxidation of ferrous iron. An attempt has been made to find whether this reaction is inhibited due to the presence of solid particles and/or the release of inorganic components from solids during coal biodesulphurization. Three types of suspended particulates (pyrite, coal and activated charcoal) and their filtrates were used in this study. The presence of pyrite and coal as suspended particulates inhibited the bacterial iron oxidation to a greater extent as compared to the inert solid (activated charcoal powder). The inhibition at 2% pulp density in case of pyrite, coal and activated charcoal was 96%, 33% and 10%, respectively. The filtrates of pyrite and coal also caused significant inhibition while activated charcoal filtrates were non-inhibitory. At higher pulp densities (5-10%) of coal particulates, the contribution of soluble components to the total inhibition of iron oxidation was more pronounced. The inhibitory concentrations of nickel (1.0 g/l), lead (0.3 g/l), zinc (9.5 g/l), aluminum (2.5 g/l) and silicon (0.2 g/l) for bacterial iron oxidation were worked out and compared with those present in the actual leachates (produced during coal biodesulphurization). Based on the results, silicon turned out to be the main suspect responsible for retarding the central step of iron oxidation during coal biodesulphurization. Consequently, operational strategies, which tend to minimize the concentration of toxic inorganic components in the leachate, could be better option than conventional batch process for enhanced biodesulphurization. (C) 2004 Elsevier B.V. All rights reserved.