With the objective of developing a sulfur-producing flue gas desulfurization (SP-FGD) process, effects of O2 and H2 O on carbothermal reduction of SO2 by oil sand fluid coke were studied by experiments and thermodynamic analysis. It was observed that the addition of O2 and H2 O enhanced SO2 conversion, which was attributed to the reducing gases, CO and H2 . Adding O2 and/or H 2 O may increase or decrease sulfur yield depending on the availability of SO2 . Thermodynamic analysis showed that C/O ratio was a key parameter in controlling equilibrium products of C-O-S and C-O-S-H systems. Results of thermodynamic analysis suggested that gas-solid reactions were slow enough to allow a gas phase equilibrium. Shrinking Core Model (SCM) analysis showed that gas-solid reactions were rate-limiting at 700°C. SEM analysis confirmed the existence of an active layer. Sulfur was found to accumulate in the porous layer. H 2 O addition had a positive effect on sulfur accumulation.