Surface modified silica particles are used as external additives to toners to improve free flow, to reduce agglomeration of toner particles, and to modify the tribo-charging characteristics of the final toner formulation. In many of the new toner formulations, large diameter spherical silica particles, rather than the traditional aggregated fumed silicas, are needed to give the required performance characteristics to the toner. In fact, large (order 100 nm) spherical silica particles have been cited as an essential additive in preventing agglomeration in new toner formulations [1].The manufacture of large treated spherical silica particles presents a variety of chemistry and processing challenges. Both the reaction and drying conditions during manufacture must be carefully controlled to produce powders with uniform treatment and minimal aggregation. In this paper, we shall discuss the effects of the treatment chemistry and the base silica particle size on toner tribo-charging and humidity stability and toner free flow for a model toner system.