Basic physico-chemical data of HI-H2O-I-2 (HIx) solution are very important for thermochemical water-splitting iodine-sulfur cycle aiming at large scale hydrogen production. HI, I-2, and H2O make up a highly non-ideal solution system, whose viscosity and the variation with composition/temperature are very essential for the flowsheet work and related CFD research. Detailed and systematic studies on HIx's viscosity are conducted in this work. The dynamic viscosity values of 53 HIx samples with different compositions are measured at temperatures ranging from 20 to 120 degrees C. The HI/H2O molar ratio of these HIx solutions ranges from 1:4.337 to 1:11.979, while the HI/I-2 molar ratio from 1:0 to 1:4.052. In the range of this work, the HIx's viscosity value changes within 0.3710-3.9085 mPa.s. HIx solution's viscosity varies dramatically when changing HI or I-2 contents. Increasing either HI or I-2 concentration will cause increase of viscosity. When heated, HIx's viscosity drops in a large scale, especially in the range of 20-80 degrees C. Polynomial regression is conducted to model the relationship among viscosity, composition and temperature. The obtained approximate quadratic polynomial model has good accuracy to reproduce most of the experimental viscosity values within a deviation of +/- 10%. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.