This paper examines the validity of the plasticity index (PI) as a criterion for estimating the liquefaction potential of clayey soils under cyclic loading. The results of undrained cyclic stress-controlled ring-shear tests on artificial mixtures of sand with different clays saturated with water indicated that an increase in PI decreased the soil potential to liquefy, and soil with PI>15 seemed to be nonliquefiable, a finding that is in agreement with the results of other researchers. However, in this study some deviations from this relation were found when a bentonite.sand mixture was treated with solutions of different ions, thus bringing into question the effectiveness of PI as a measure of the liquefaction potential of clayey soil having a certain pore water chemistry.