The modified Claus process is the most common method for the conversion to sulphur of the hydrogen sulphide contained in sour oil and natural gas. An important but relatively unstudied part of the Claus process are the reaction kinetics of the front-end reaction furnace in which sulphur production takes place, hydrocarbon contaminants are destroyed and reactions occur that prepare the sour gas for downstream catalytic processing. One of the key reactions that occurs in the front-end furnace is between H2S and SO2. This second part of the Claus reactions has been studied under catalytic but not thermal conditions. The purpose of this work was to study this reaction at actual Claus plant reaction furnace temperatures and residence times. The new kinetic data would then be used to develop a new reaction rate expression. Experiments were performed in a laboratory scale, isothermal, plug-flow reactor at temperatures between 850 and 1150 degreesC and at residence times between 0.05 and 1.2 s. Overall conversion of H2S and SO2 were measured and results are presented in this paper. The newly developed kinetic rate expression is as follows: r = A(f) exp(Eur/RT) P-H2S P-SO2(0.5) - A(r) exp(Eur/RT) P-H2O P-S2(0.75), where A(f) = 15,762(+/- 1200) mol cm(-3) s(-1) atm(-1.5), E-af = 49.9(+/- 0.3) kcal mol(-1), A(r) = 506(+/- 50) mol cm(-3) s(-1) atm(-1.75) and E-ae = 44.9(+/- 0.5) kcal mol(-1). The new rate expression correlates experimental H2S and SO2 conversion data within 12 and 18%, respectively. In addition, the predicted conversion for the new rate expression extrapolates correctly to equilibrium conversion values and the Arrhenius parameters predict the heat of reaction to within 0.05%.