Sulfur-oxidising acidophilic bacteria were obtained from weathered sulfur piles produced by a petroleum refinery. When grown on commercial sulfur the yield was 10(10) cell/g S. Sulfur conversion to sulfate was about 95% after 17 days. Cultures were also grown together with ash obtained from incinerated refinery sludge, which contained high amounts of iron. Cultures grown in ash plus sulfur gave somewhat higher values for the growth parameters (Y = 1.6 x 10(10) cell/g S). The sulfur conversion was about 70% (after 17 days) and more than 90% of the iron present in the ash was also leached. The sulfur-reduced compound thiosulfate, determined using ion pair HPLC, was found in the presence and absence of ash although the profile was different in each case. Sulfite was only found in non-ash containing cultures, whereas tetrathionate was only formed in the presence of ash. These results are discussed with reference to the substrates used by the micro-organisms. (C) 2008 Elsevier Ltd. All rights reserved.