The force needed to detach five sets of different size toner particles, having number averaged diameters between 3.6 and 8.5 μm, from a composite aggregate photoconductor was measured using an ultracentrifuge. In addition to size variations, each toner size had four different levels of silica concentration, adjusted so that the surface area concentration at each level was kept constant for the five toner sizes. Due to the changing silica concentration and particle size, toner charge also varied. It was found that the dependence of detachment force on toner charge was small, with the correlation actually being slightly negative. However, the detachment force increased monotonically with increasing toner diameter and decreased monotonically with increasing silica concentration. Moreover, upon normalizing the detachment force to the toner diameter and the silica concentration to the surface area concentration of silica, it was found that the detachment force clustered into groups in which the force needed to separate the toner from the photoconductor depended only on the silica concentration. These results suggest that van der Waals interaction, rather than electrostatic forces, are the dominant mechanism controlling toner adhesion in this instance.