In the SAGD (Steam-Assisted Gravity Drainage) process, high temperature saturated-steam is injected into an oil sands reservoir to lower the viscosity of the bitumen hosted within the reservoir. Typical steam temperatures range from 200 to 260 degrees C. Under these conditions, the bitumen, in presence of steam-condensate, undergoes aquathermolysis yielding H2S and CO2. Current SAGD simulation models take into account complex spatial heterogeneity of geology and oil composition and the physics of heat transfer, multiphase flow, gas solubility effects, and viscosity variations with temperature; however, few have taken the chemistry of SAGD into account. Here, we have added aquathermolysis reactions to a simulation model to understand H2S reactive zones in SAGD. Given the requirement to constrain sulfur emissions from thermal recovery processes, it is necessary to reduce H2S production to surface. The results demonstrate that injecting small amount of SO2, along with steam, initiates the Claus reaction in the reservoir which converts H2S into liquid sulfur. Thus, sulfur emissions are reduced below that of the original operation without SO2 co-injection. The ability to use in situ Claus reaction-based H2S scavenging offers an elegant way to reduce sulfur emissions from thermal oil sands recovery processes. (C) 2013 Elsevier Ltd. All rights reserved.