The species present in the condensed phase of the HIx, 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(-1) in samples of the two aqueous binary systems, I-2/H2O and HI/H2O, and the ternary system HI/I-2/H2O with and without the addition of H2SO4 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 20-300 degrees C. 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/H2O system, reveal the co-occurrence of H+I3-, H+-I (I-2), and H+-I (I-2)(2) solvated species in the condensed phase of HIx. Thus, while the first is mostly evident by its strong fundamental band with Raman shift in the range from 110 to 115 cm(-1), the other two species appear convoluted in a broad prominent band whose Raman shift ranges between 153 and 172 cm(-1) 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. Copyright (C) 2010 John Wiley & Sons, Ltd.