Supercritical water (SCW) has been used as a reaction medium, primarily for the oxidation of hazardous chemical wastes. It is essential to obtain the hydrothermal reaction kinetics and provide data for the intermediate products if the supercritical water oxidation process is to be developed. In this paper, the hydrothermal decomposition rate of thiodiglycol [(HOC2H4)2S, TDG], as the model compound of HD mustard, was investigated in an isothermal continuous tubular reactor under SCW conditions. The reaction temperatures ranged from 386 to 528°C and the residence times varied from 13 to 87 s at a fixed pressure of 25 MPa. The conversion of TDG was monitored by analyzing the total organic carbon (TOC) content of the liquid effluent samples. By using a first-order global power-law reaction rate expression, the hydrothermal decomposition rate of TDG, based on the TOC disappearance rate, in SCW was regressed from experimental data to a 95 % confidence interval. The resulting activation energy was 71.32±5.20 kJ/mol and the pre-exponential factor of the reaction was 5.57(±0.002)×10³s-1.