Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H2S, SO2, and S2O such as might occur in the front-end furnace of the Claus process. The direct reaction between H2S and SO2 occurs via a 5-centered transition state with an initial barrier of similar to 135 kJ mol(-1) and an overall barrier of similar to 153 kJ mol(-1) to produce S2O and H2O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by similar to 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H2S and S2O can occur via two possible pathways; the analogous reaction to H2S + SO2 has an initial barrier of similar to 117 kJ mol(-1) and an overall barrier of similar to 126 kJ mol(-1) producing S-3 and H2O, and a pathway with a 6-centred transition state has a barrier of similar to 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.