Platinum nanoparticles finely dispersed in activated carbon powders have been observed with high contrast in secondary electron (SE) and backscattered electron (BE) images. The factors that contribute to the visibility of small metal particles in high-resolution BE and SE images are discussed. Monte Carlo simulations provided insight into the scattering of electrons by small, heavy-element particles located on, or embedded in, light-element supports. The visibility of Pt nanoparticles in BE images depends on many factors including the size and the location of the Pt particles, the energy of the incident electrons, and the size of the electron probe. The SE signals generated by backscattered electrons may not significantly contribute to the visibility of small Pt particles in high-resolution SE images of carbon supported Pt catalysts. Only those small Pt particles that are located on or very close to the surface of the carbon support can be revealed in high-resolution SE images.