This paper describes the results of an experimental investigation aimed at establishing whether pH affects the cyclic strength of artificial and natural fine-grained soils. The undrained cyclic behavior of low plasticity (liquid limit less than 30) kaolin and illite-sand mixtures, medium plasticity (liquid limit less than 50) bentonite-sand mixture, and medium plasticity natural soil recovered from the sliding zone of an earthquakeinduced landslide was studied by means of a ring-shear apparatus with a view to understanding the structural changes that occurred at different pH. The data obtained for the low plasticity clay-sand mixtures indicated that changes in pH could slightly affect the soil resistance to liquefaction. For the bentonite-sand mixture of medium plasticity, the results not only revealed the strong influence of pH but they also demonstrated that in acidic and alkaline mediums the cyclic strength of soil significantly decreased. The outcome of tests on the natural soil corroborated the tendency observed for the bentonite-sand mixture, thus emphasizing the importance of pH in the cyclic behavior of medium plasticity clays. An attempt was made to understand the cause of the remarkable pH-dependent change in cyclic strength of the studied soils based on the principles of colloid chemistry.