A method for determining thiodiglycol (TDG), a mustard gas hydrolysis product in water, serum and urine samples using gas chromatography-mass spectrometry (GC-MS) after tert-butyldimethylsilylation (TBDMS) is described. Quantitation of TDG was performed by measuring the respective peak area on the extracted ion chromatogram of m/z 293, using an internal standard, the TDG homologue, thiodipropanol, peak area of which was measured as m/z 321. The presence of salts in the sample solution not only suppressed the loss of TDG by vaporization during the evaporation of water, but also facilitated the rate of production of di-silylated derivative, bis(tert-butyldimethylsilyoxylethyl)sulfide (TDG-(TBDMS)2). Under the pretreatment conditions used, in which 0.5 ml of water sample supplemented with 100 microM potassium chloride was evaporated to dryness under reduced pressure, followed by reaction with N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide at 60 degrees C for 1 h, TDG-(TBDMS)2 was reproducibly detected with about a 55% recovery and a limit of detection (LOD, scan mode, S/N = 3) of 5.4 ng/ml. TDG was also determined by GC-MS from a 0.5 ml serum sample (after perchloric acid deproteinization) and from a 0.1 ml urine sample, after TBDMS derivatization. The LOD was determined to be 7.0 and 110 ng/ml for serum and urine, respectively.