The hydration of clay minerals due to adsorption of water molecules and hydrated cations in electrolyte aqueous solutions would affect the stability of dispersions, resulting in some detrimental effect in mineral processing and hydrometallurgy. A parameter, called as hydration index I, was presented to evaluate hydration layers on fine clay mineral surfaces in different electrolyte aqueous solutions through measurements of the relative viscosities of fine mineral dispersions as a function of the volume fractions of dry particles. The effect of type and concentration of electrolyte on hydration of fine clay mineral particles in aqueous solutions was investigated. The results showed that both hydration index I of montmorillonite and kaolinite in Na+ aqueous solutions were larger than that in Ca2+ aqueous solutions at the concentration of 0.001 mol/L; either in Na+ or Ca2+ aqueous solutions, the hydration index I of montmorillonite increased firstly with increasing concentration of electrolyte in aqueous solutions, then decreased when being above a certain concentration, whereas the hydration index I of kaolinite decreased with increasing electrolyte concentration when it being above 0.001 mol/L. (C) 2015 Elsevier B.V. All rights reserved.