The individual adsorption events of sub-mu m silica and polystyrene spheres (310-530 nm in diam.) were detected by monitoring the blocking of redox mediator diffusion to Pt ultramicroelectrode (UME) substrates by the adsorbing spheres. Under the diffusion limited oxidation of FcMeOH and at low supporting electrolyte concentrations, the negatively charged spheres arrive at the electrode by electrophoretic migration. Sphere adsorption monitoring experiments consisted of long-time (1000-5000 s) chronoamperograms recorded in solutions with fM concentrations of spheres and different concentrations of supporting electrolyte. Trends in the heights of the step features with time reflect changing surface coverage of spheres, and coupled step features in the chronoamperograms suggest dynamic rearrangement of spheres on the surface. Numerical simulations of diffusion blocking at electrodes by adsorbing particles as well as mass transport of particles under migration were also performed, and show good agreement with the experimental data collected.