Ion hydration is critically important to many biological applications, including protein unfolding. However, the hydration of ionic liquids (a new type of organic salts) is not well understood. One property for characterizing the effect of ion hydration is kosmotropicity. To quantify the ion kosmotropicity, this review evaluated several thermodynamic parameters including B-coefficients, structural entropies, structural volumes, and ion mobility. The availability and reliability of experimental data and various models for these parameters were examined for ionic liquids. The relationship and comparison of these parameters are also discussed. Smaller pyridinium and imidazolium cations in ionic liquids are more likely to be chaotropes or borderline ions owing to their hydrophobic hydration. The kosmotropicity of anions varies. (c) 2006 Society of Chemical Industry.