Platinum nanoparticles were electrochemically deposited onto highly oriented pyrolitic graphite (HOPG) from H2PtCl6 solutions and observed by tapping mode atomic force microscopy. Spontaneous Pt deposition, which resulted in a wide particle size distribution, would occuron HOPG at open-circuit potential but could be suppressed by using anodic bias of the substrate before and after deposition. Nanoparticles with a narrow size distribution could be obtained when spontaneous reduction was avoided. Pt nucleated both at step edges and on terraces, with a preference for the former. The density of Pt nanoparticles on HOPG was 10(9)-10(10) cm(-2). Increasing the deposition overpotential or adding HCl as supporting electrolyte resulted in more uniform particles and less aggregation. These findings confirm previous results obtained by our group using only electrochemical methods.