SUMMARYThe species present in the condensed phase of the HI x , hydrogen producing, feed in the water-splitting, sulfur-iodine thermochemical cycle have been investigated using spontaneous Raman scattering. Measurements of I 2 -containing species in the low Raman-shift region from 50 to 400 cm À1 in samples of the two aqueous binary systems, I 2 /H 2 O and HI/H 2 O, and the ternary system HI/I 2 /H 2 O with and without the addition of H 2 SO 4 have provided a consistent picture of the aqueous iodine and polyiodide chemistry. Samples were contained in sealed silica ampoules and were heated to temperatures in the range 20-3001C. The results, which cover a wide range of I 2 and HI mole fractions, and x I2 =x HI mole ratios, in the HI/I 2 /H 2 O system, reveal the co-occurrence of, and H 1 -I À (I 2 ) 2 solvated species in the condensed phase of HI x . Thus, while the first is mostly evident by its strong fundamental band with Raman shift in the range from 110 to 115 cm À1, the other two species appear convoluted in a broad prominent band whose Raman shift ranges between 153 and 172 cm À1 depending on the x I2 =x HI mole ratio. These well characterized Raman features are proposed as an in situ diagnostic for process control of the cycle.

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