The force needed to detach spherical toner particles having a number average radius of 7.1 μm from an organic photoconductor was determined by ultracentrifugation. In the absence of any release agents applied to the photoconductor, it was found that only a small fraction of the toner particles could be removed from the photoconductor, even at the highest centrifugal accelerations (354,000 g). However, when the photoconductor was coated with a thin layer of zinc stearate, the release force was reduced substantially and detachment was readily achieved, the remaining release force varying with the square of the toner charge-to-mass ratio. Hence, the residual detachment force varied as the square of the particle charge. These results suggest that electrostatic forces become dominant when van der Waals forces are greatly reduced. Conversely, the large increase in toner adhesion to a photoconductor observed in the absence of a good release agent suggests that van der Waals forces may often dominate toner adhesion for this size particle.