A new method for determination of thiosulfate in human urine has been developed and validated. Analytical procedure is very simple and consists of only few steps: derivatization of thiosulfate with 2-chloro-1-methylquinolinium tetrafluoroborate, centrifugation of a mixture, separation of so-formed derivative bymicellar electrokinetic chromatography with sweeping and UV detection at 375 nm. A fused-silica capillary with an inlet to detector length of 51.5 cm and a total length of 60 cm (75 mu m id) was served as a separation column. The separation voltage of 20.5 kV (similar to 160 mA) and buffer solution consisting of 0.055 mol/L sodium phosphate (pH 8), 25% acetonitrile, and 0.035 mol/L sodium dodecyl sulfate were found to be the most suitable conditions for the effective separation. The limit of quantification for thiosulfate was 4 mu mol/L urine. The method was validated and calibrated for thiosulfate in the range of 4-64 mu mol/L (R-2 = 0.9997). The relative standard deviation of the points of the calibration curve varied from 1.2 to 4.8% RSD.