The effects of surface area and surface modification of amorphous fumed silicas on the charging properties of styrene-butadiene toner, against polymer-coated metal beads, were studied systematically. Without exception, silica additives though used at very low concentration (0.5% by weight on the toner surface) dominated the charging of the resulting toners. At 20% RH (relative humidity), our data showed that (1) all silicas impart negative charging in the styrene–butadiene toner, (2) hydrophobic silicas charge more negative than hydrophilic silicas, (3) the charge level is primarily governed by the surface area of the silica, and (4) the rate of charge exchange as measured by the admix time increases as the hydrophobicity of the silica increases. The effect of RH on toner charging was also examined. Results indicated that charging is universally sensitive to RH when silicas are used in toners. Specifically, tribo decreases as RH increases. Hydrophobic silicas were more resistant to RH changes, presumably because of their smaller moisture uptake at high RH. Accompanying the decrease in tribo, a reduction in admix time was generally observed at high RH. Use of the knowledge gained in this work for future toner formulation is suggested.