Response surface methodology was applied to determine the optimal medium composition for sulphuric acid production by an indigenous Thiobacillus thiooxidans. With a view to reducing the number of experiments and obtaining the mutual interactions between the variables, a 2(4) full-factorial central composite design was employed for experimental design and analysis of the results. Four independent variables, KH2PO4, (NH4)(2)SO4, MgSO4 and elemental sulphur, in the growth medium were tested. Elemental sulphur was found to exhibit positive correlation with the sulphuric acid yield, whereas the other three variables showed no significant effect. No mutual interactions were found between elemental sulphur and each of the other three variables. The optimal composition of the growth medium to achieve the optimal production of sulphuric acid was determined as follows (g/l): KH2PO4 = 3.5, (NH4)(2)SO4 = 4.9, MgSO4 = 0.7 and elemental sulphur = 23.7. The corresponding sulphuric acid production and dry cell weight were 38662 ppm and 0.7 g/l, which are about 2.6- and 1.75-fold increase, respectively, compared with those using the conventional T thiooxidans optimum growth medium. (C) 2003 Elsevier Ltd. All rights reserved.