In thermochemical water-splitting iodine-sulfur cycle for hydrogen production, basic physico-chemical data of HI-H2O-I-2 (HIx) solution are very important. Detailed and systematic studies on density/coefficient of thermal expansion (CTE) are in great need. In this work, the density values of 53 HIx samples with different compositions are measured at atmospheric pressure and temperatures ranging from 20 to 90 degrees C. HIx solution's density varies dramatically when changing HI or I-2 contents. Increasing either HI or I-2 concentration will cause increase of density. When heated, HIx's density decreases because of thermal expansion. With the help of density-temperature curves, the CTE values are calculated for HIx solutions of different compositions. It is found that increase of either I-2/HI or H2O/HI will bring rise of HIx's CTE. Although the CTE value is relatively small, it is very sensitive to the change of composition. In the range of this work, the HIx's CTE value changes within 5.45E-4/degrees C to 9.17E-4/degrees C. Polynomial regression is conducted to model the relationship between CTE and the composition. The obtained approximate quadratic polynomial model has good accuracy to reproduce most of the experimental CTE values within a deviation of +/- 5%. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.