Xerographic toners use submicron metal oxide surface additives to control powder flow. Indeed, oxides have been used to improve flowability or compressibility of cohesive powders for decades, from coffee whitener to aspirin. To date there has been little attempt to understand the mechanism of action of flow aids. In this paper, toner powder cohesion was measured with respect to the primary particle size, hydrophobicity, and hydrogen bonding of the oxide. Cohesion is interpreted in terms Van der Waals, capillary, and hydrogen bonding forces. Flow with oxides is determined by the nano-geometry of contact between oxide particles, determined by the oxide primary particle size. Adhesion forces decrease with decreasing oxide primary particle size and increasing hydrophobicity.