The forces needed to remove irregularly-shaped. 8 mum diameter, polyester particles from a polyester substrate were measured using an ultracentrifuge. Measurements were also made on a second set of similar particles where nanometer-size silica clusters had been placed on their surfaces. These silica clusters acted as spacers, reducing direct contact between the particle and the substrate. It was found that the separation forces for the bare particles were consistent with predictions of the JKR theory of adhesion, but were much larger than could be accounted For from simple electrostatic interactions associated with either uniformly-charged particles or particles with localized charged parches. It was found, however, that the forces needed ro effect separation decreased with increasing silica concentration. For particles with 2% by weight silica clusters on their surfaces, the separation force was only about 5% of the separation forces of the bare particles. At this concentration of silica, the estimates of the separation forces obtained from JKR theory, from the uniformly-charged model, and From the localized-charged-patch model are all about equal. The numerical estimates are consistent with the experimentally-obtained values.