The Sulphur-Iodine (S-I) thermochemical cycle for hydrogen production from water is one of the widest investigated cycles in the world. The S-I cycle is composed of three main sections which correspond to its three main reactions: the Bunsen reaction, the hydriodic acid (HI) decomposition and the sulphuric acid decomposition. The investigation, comprehension and modelling of the complex electrolytic chemistry of the HI solutions is crucial for the process optimization. In particular, the analysis of chemical speciation in the HI solutions is a key factor for the full understanding of the chemical equilibria involved at varying 12:HI ratio in the mixtures. The aim of the present study is to use Raman spectroscopy for characterizing the chemical speciation of iodine compounds existing in the mixtures that pertain to the S-I process. For this purpose, Raman spectra of solutions with known HI and 12 concentration have been recorded. It has been found that the chemistry of these solutions is highly dependent on 12:HI molar ratio. Up to 1:1 ratio, I and its corresponding ion pair HI3* are the dominant iodine compounds. At higher values, the formation of higher polyiodine and polyiodides compounds has been suggested. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.