The influence of interface properties of silicon nitride powder on the dispersion and particle size distribution in aqueous environments was investigated in the presence of cationic and anionic polyelectrolytes. The isoelectric point of as-received silicon nitride powder at pH 6.6 shifted to 4.0 in the presence of 1500 ppm ammonium polymethacrylate, an anionic polyelectrolyte (APE). However, no isoelectric point could be observed up to pH 10.8 when 1500 ppm quaternized polyamine epoxychlorohydrin, a cationic polyelectrolyte (CPE), was present in the silicon nitride suspensions. The dispersion characteristics of silicon nitride powder in the presence of various concentrations of the two polyelectrolytes and by pH variation were examined. The slightly better dispersion of silicon nitride powder in the presence of CPE than that in the presence of APE is explained in terms of differences in the adsorption characteristics and surface charge concentration in aqueous environments. A mechanistic understanding of the role of interface chemistry and, in particular, polyelectrolyte adsorption on the dispersion of silicon nitride powder is presented.