Xerographic toners use sub-micron metal oxide surface additives to control powder flow. Toner powder cohesion, which is inversely related to toner flow, was measured with respect to the primary particle size and surface chemistry of the oxide used as the flow additive. The cohesion data is interpreted in terms of a quantitative cohesive force mechanistic model for toner flow. Thus, cohesion is described in terms of Van der Waals, capillary, and hydrogen bonding forces between the oxide particles on the toner surfaces, and by the nanogeometry of the contact between oxide particles, controlled by the oxide primary particle size. Adhesion forces decrease, and thus flow improves, with decreasing oxide primary particle size and with increasing hydrophobicity.