Two methods have been developed for the analysis of thiodiglycol sulphoxide, a metabolite of sulphur mustard, in urine. The first method recovers thiodiglycol sulphoxide from urine by extraction from a solid absorbent tube and clean up on Florisil. In the second method thiodiglycol sulphoxide is reduced to thiodiglycol with acidic titanium trichloride prior to extraction. This method detects thiodiglycol, thiodiglycol sulphoxide, and their acid-labile esters, as the single analyte thiodiglycol. In both cases the recovered analytes were converted to the bis(pentafluorobenzoyl) derivative of thiodiglycol and detected by gas chromatography-mass spectrometry using negative ion chemical ionisation. The limits of detection were 1 ng per 0.5-ml sample of urine. Urine from five normal human subjects showed low background levels of thiodiglycol sulphoxide in the range 2-8 ng/ml. However, a sixth subject was found to be excreting levels of thiodiglycol sulphoxide as high as 36 ng/ml. The first method has been used in toxicokinetic studies of sulphur mustard and the second method is intended to be used for the retrospective confirmation of mustard poisoning in casualties of chemical warfare.