In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H 2SO 4 phase and HI x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I 2 molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I 2 solution from the HI x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I 2 solution from the distillation column decreased the flow rate of I 2 solution fed to the distillation column from the separator. The variation of I 2 molar fraction in the H 2SO 4 phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I 2 solubility, 2-liquid phase disappearance and SO 2 reaction amount.