We have studied the effects of an external additive on the non-electrostatic and electrostatic adhesion of toner. Toner samples with various concentrations of the additive were prepared. The additive surface coverage on the toner was evaluated with a scanning electron microscope. The adhesion to a photoconductor of non-tribocharged or tribocharged toner was measured by the centrifugal method. It was found that the non-electrostatic adhesion rapidly decreased with increasing surface coverage, and saturated at a certain coverage. The dependence of the non-electrostatic adhesion on the surface coverage can be explained as a change of the van der Waals force. The electrostatic adhesion increases in proportion to the square of toner charge to mass ratio, and the slope decreases exponentially with increasing surface coverage. A simple electrostatic image force model cannot explain an enhanced electrostatic adhesion. It is suggested that the enhanced adhesion is caused by a non-uniform charge distribution in the toner surface. The decrease in electrostatic adhesion with increasing surface additive is attributed to increasing total charged area on the toner particle.