The Rebinder Effect, an environment-caused change in the hardness of rock, ceramic, or glass, was investigated in silicon nitride using conventional microhardness and electrochemical techniques. As in previous studies of the effect, hardness and zeta potential varied significantly with the pH of the environment. Unlike previous studies, the pH values at which hardness was maximum and zeta potential was zero (zero point of charge, or zpc) did not correspond. This lack of correspondence was deemed the result of the partial oxidation of the surface film of the sample. The presence of the oxide containing film had a significant effect on zeta potential but little effect on hardness as the hardness indenter penetrated well beyond the film. Through the use of a simple linear model and published data it was possible to demonstrate that for oxide-free silicon nitride the pH values at which hardness was maximum and zeta potential was zero do correspond. The Rebinder Effect in this material is clearly time-dependent; long dwell times resulted in reduced hardness values. This was more noticeable at pH values removed from the zpc. These observations are consistent with a mechanism for the Rebinder Effect in which environmental pH controls surface charge, which in turn controls zeta potential and affects dislocation motion and, thereby, hardness.