The determination of thiodiglycolic acid (TDGA) In urine by gas chromatography (GC) is described. The analytical procedure includes addition of internal standard (o-phthalic acid), ethyl acetate extraction, evaporation of the solvent and silylation of TDGA with N-trimethylsilyl-diethylamine in pyridine (1:1). The results are calculated from the ratio of TDGA and internal standard peak heights. The detection limit of the method is 10 mg/l and the coefficient of various is +/- 5%.