Abstract Hydrophobically modified fumed silica particles are widely used in toner formulations to enhance toner free flow and triboelectric charging. Despite the important role that dielectric powders have in electrophotography, the triboelectric charging of such powders is still poorly understood. In this article, the authors present experimental data on the charging properties of toners formulated with fumed silica additives that were modified with different levels of fluoroalkylsilanes. The results show that at high ambient relative humidity (RH) conditions, the triboelectric charge correlates with the fluorine content of the additive, whereas at low RH, the triboelectric charge is effectively constant with respect to fluorine content. The same result was obtained with two different types of toner particles. The authors attempt to rationalize these results in terms of surface conductivity at high RH conditions and charge saturation at low RH conditions. They also present a series of zeta potential measurements that show a correlation with tribocharge at high RH and interpret the results in terms of the ion-transfer model for triboelectric charging.