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In the integrated gasification combined cycle (IGCC) process, the sulfur compounds present in coal are converted to hydrogen sulfide (H₂S) when the coal is gasified. Due to its harmful effects on sorbent/solvent and environmental regulations, H₂S needs to be removed from the product gas stream. To simulate the H₂S removal process, desulfurization was carried out using a dry sorbent as a fluidizing material within a bubbling, high-temperature fluidized bed reactor. The ZnO-based sorbent showed not only an excellent capacity of H₂S removal but also long-term stability. However, unexpected SO₂ gas at a concentration of several hundred ppm was detected during the desulfurization reaction. Thus, we determined that there is an unknown source that supplies oxygen to ZnS, and identified the oxygen supplier through three possibilities: oxygen by reactant (fresh sorbent, ZnO), byproduct (ZnSO₄), and product (H₂O). From the experiment results, we found that the H₂O produced from the reaction reacts with ZnS, resulting in SO₂ gas being generated during desulfurization. The unknown oxygen source during desulfurization was deduced to be oxygen from H₂O produced during desulfurization. That is, the oxygen from produced H₂O reacts with ZnS, leading to SO₂ generation at high temperature.