A study has been made of the kinetics of the thermal reduction of S(IV) acid tars of the Parex process under isothermal conditions and with a uniform increase in the reaction temperature. It has been shown that the absence of excess reducing agents makes the complete conversion of H2SO4 and sulphonic acids of the initial mixture to SO2 impossible. A kinetic equation of the process is proposed, describing the liberation of sulphur dioxide in the entire range of variation in the temperatures of first-order reactions.