A scanning electron microscope with a short-focal-length "immersion" lens and subnanometer resolution has been used to characterize several oxide-supported metal particle catalysts. Nanometer-sized metal particles in the Pt/TiO2 and Pd/SiO2 systems could be imaged with best clarity at the upper end of the operating voltage range (20-30 kV). However, visibility depended upon an adequate yield of secondary electrons relative to the support : small Pt particles on CeO2 could not be located by secondary electron imaging. Best visibility of the surface topography of the support was obtained at lower accelerating voltages.