The collisions of single platinum nanoparticles at an ultramicroelectrode were observed electrochemically by their characteristic current-time transients for a particle-catalyzed reaction. A single event is characterized by the current generated by an electrocatalyzed reaction of an indicator species (proton, hydrogen peroxide) present in solution. Since the indicator reaction does not occur at the selected ultramicroelectrode and can involve a high concentration of indicator species with a much larger diffusion coefficient than the nanoparticle, large amplification (10 orders of magnitude or more) in the current occurs. Every collision produces a unique current-time profile that can be correlated with the particle size, the particle residence time, and the nature of the particle interaction with the electrode surface. Applications to studying heterogeneous kinetics at single nanoparticles, determining particle size distributions, and as a very sensitive electroanalytical technique are suggested.