To isolate thiodiglycol (TDG)-degrading bacteria, the mustard gas hydrolysis product, and to characterize the metabolites formed and the enzymes involved in the degradation. Two strains, identified as Achromobacter xylosoxydans G5 and Paracoccus denitrificans E4, isolated from a petroleum-contaminated soil, utilized TDG as sole carbon and sulfur source. During the degradation of TDG by strain E4 [(2-hydroxyethyl)thio] acetic acid (HETA), thiodiglycolic acid (TDGA) and bis-(2-hydroxyethyl)disulfide (BHEDS) were identified by gas chromatography-mass spectrometry analysis, while HETA and TDGA were identified for strain G5. Two-dimensional isoelectric focussing-gel electrophoresis (2-D IEF/SDS-PAGE) maps of protein extracts of P. denitrificans E4 grown on TDG showed a spot identified as a methanol dehydrogenase. Increased expression of a putative iscS gene, involved in sulfur assimilation, was observed in TDG-grown cells of A. xylosoxydans G5. TDG degradation by P. denitrificans E4 occurred through two pathways: one involved cleavage of the C-S bond of HETA, yielding BHEDS and the other, oxidation of the alcoholic groups of TDG, yielding TDGA. The cleavage of the C-S bond of TDGA gave mercaptoacetic acid, further oxidized to acetate and sulfate. Increased knowledge of TDG-degrading bacteria and the possibility of using them in a tailored-two-stage mustard gas destruction process.