A dibenzothiophene (DBT) sulfone monooxygenase (TdsA), which catalyses the oxidative C-S bond cleavage of DBT sulfone to produce 2-(2-hydroxyphenyl)benzenesulfinate (HPBS) was purified from the thermophilic DBT desulfurizing bacterium Paenibacillus sp. strain A11-2 by multistep chromatography. The molecular mass of the purified enzyme was determined to be 120 kDa by gel filtration and the subunit molecular mass was calculated to be 48 kDa by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) indicating a dimeric structure, The N-terminal amino acid sequence of the purified TdsA was determined to be MRQMHLAGFFAAGNTHH, which revealed no significant similarity to any other known amino acid sequences. The purified TdsA absolutely required an oxidoreductase for its activity. This oxidoreductase (TdsD) was also purified to homogeneity, and its molecular size was calculated to be 50 kDa and 25 kDa by gel filtration and SDS-PAGE, respectively. TdsD was completely FMN-dependent, and FAD could not act as a cofactor. The N-terminal amino acid sequence of the purified TdsD was determined to be TSQTAEQSIAPIVAQYRHPEQPISALFVNR, which showed significant similarity to kinesin-like protein (44% identity), The optimal temperatures for the activity of TdsA and TdsD were 45 degreesC and 55 degreesC, respectively. Both enzymes showed optimal activity at pH 5.5, TdsA was slightly inhibited by sulfate, but not by 2-hydroxybiphenyl (2-HBP), which is another end product of DBT, TdsA showed higher activity toward bulkier substrates than its mesophilic counterpart, DszA, These properties suggest the applicability of biodesulfurization to the processing of actual petroleum fractions.