The Sulfur-Iodine (SI) thermochemical hydrogen production process is one of the most promising thermochemical water splitting technologies. In the integrated operation of the SI process, the O2 produced from a H2SO4 decomposition section could be supplied directly to the Bunsen reaction section without preliminary separation. A HIx (I2+HI+H2O) solution could be also provided as the reactants in a Bunsen reaction section, since the sole separation of I2 in a HIx solution recycled from a HI decomposition section was very difficult. Therefore, the Bunsen reaction using SO2-O2 mixture gases in the presence of the HIx solution was carried out to identify the effect of O2. The amount of I2 unreacted under the feed of SO2-O2 mixture gases was little higher than that under the feed of SO2 gas only, and the amount of HI produced was relatively decreased. The O2 in SO2-O2 mixture gases also played a role to decrease the amount of a impurity in HIx phase by only striping effect, while that in H2SO4 phase was hardly affected.