This paper elucidates the influence of alkaline solutions on shear characteristics of bentonite and bentonite–sand mixtures. The compacted Ca–bentonite and bentonite–sand mixtures were immersed in 0.1 mol/L–KOH and KOH–NaOH solutions at 40 ℃. Triaxial compression tests were executed using the immersed specimens. The mineral composition, montmorillonite content, expansivity of montmorillonite, mean layer charge, and leachable cations of the samples were investigated. The shear characteristics of the compacted bentonite changed after the immersion in alkaline solutions. The reduction of dry density, likely induced by the dissolution of minerals, and the K-exchange of montmorillonite are related to the decrease in shear strength. K-exchanged montmorillonite behaved as a pseudo-non-swelling layer. It is suggested that the precipitation of secondary products, composed of mainly calcium, contributed to the increase in shear strength. The change in the shear characteristics of the compacted bentonite was interpreted as primarily resulting from a combination of dissolution and precipitation.