An increase in the exploitation of sour reservoirs with a high content of acid gas (mainly H2S and CO2) and the enforcement of stringent emission standards have triggered demand for high sulfur recovery efficiency in sulfur recovery units (SRUs). The high amount of H2O that is present in feed gases to the SRU can act as a diluent in the thermal section of the SRU and can also pose operational difficulties and have substantial chemical effect on the equilibrium-limited reactions of sulfur production and the deStruction of impurities. This paper explores the role of H2O (0-12% in the furnace feed). in sulfur recovery and in the fate of undesirable aromatics, COS, and CS2 in the thermal section using a detailed reaction mechanism and industrial SRU operating parameters. An intreasing H2O concentration in the feed decreased the sulfur recovery efficiency in the thermal section significantly, as the equilibrium conversion of H2S to sulfur was affected. The higher H2O concentrations increased the concentrations of H2S H-2, and SO2 at the furnace exit because of the production of OH radicals from enhanced chemical decomposition of H2O by active radicals such as SH, O, and H. It also increased the quantity of unwanted aromatics exiting the Claus furnace; but COS and CS2 production, declined because of the fall in the decomposition of CO2 and feed hydrocarbons as the furnace temperature decreased. This study provides guidelines to increase sulfur production in the thermal section and reduce the number of catalytic stages to decrease SRU operational cost.