A procedure for the isotachophoretic determination of thiodiglycolic acid (TDGA) in urine after desalination and pH adjustment of analyzed samples was elaborated. The salts present in urine were precipitated by addition of ethanol. TDGA was determined in anionic isotachophoretic mode in 0.01 M HCl as a leading electrolyte containing 0.01 % (2-hydroxyethyl)cellulose at pH 4.0 (adjusted with 6-aminohexanoic acid). 0.01 M acetic acid was used as a terminating electrolyte. The reproducibility of TDGA determination was very good, the relative standard deviation was less than 1.5 % for a concentration of TDGA 90 mg.l^-1.