Abstract: Liquid–liquid equilibrium (LLE) phase behavior was investigated for the binary iodine–water (I2–H2O) and the ternary iodine–hydroiodic acid–water (I2–HI–H2O) at the elevated temperatures and pressures of interest for the reactive distillation column of the Sulfur–Iodine Cycle. A continuous-flow apparatus, with wetted parts fabricated from tantalum-tungsten alloys, was designed and constructed for the highly corrosive conditions of this work. A central feature of the apparatus is the equilibrium view cell, which allows for the observation and discernment of vapor-liquid, liquid-liquid, and liquid–liquid–vapor equilibria for HIx systems. For the I2–H2O system, LLE was observed at temperatures and pressures to 300°C and 104 bar. Based on our observations, we estimate that the upper critical solution temperature for this system occurs at 310–315°C. For the I2–HI–H2O ternary, VLE, LLE, and LLVE were all observed at 160 and 200°C, and three compositions on the water-rich side of the LLE phase envelope were determined. [Copyright &y& Elsevier]