The Rebinder effect, an environment-caused variation in the hardness of rock, ceramic, or glass, was investigated in polycrystalline aluminum oxide using conventional microhardness and electrochemical techniques. For an environment of pH-adjusted distilled water, both hardness and zeta potential were found to vary with pH. Maximum hardness and zero zeta potential were found to occur at pH 9.5. Observations in this study are consistent with a mechanism for the Rebinder effect in which changes in surface charge influence near-surface plastic deformation of surfaces and, thereby, hardness.