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The scanning tunnel microscope (STM) technique was used to investigate the effect of preparative variables on the degree of aggregation of platinum clusters on a highly oriented pyrolytic graphite (HOPG). Two methods of depositing Pt were investigated: impregnation and vapor deposition. The diameters of the Pt clusters formed on an oxidized HOPG sample prepared by impregnation range from 50 to 500 Å. The clusters were apparently concentrated along the steps and rough patches on the surface. The sizes of the Pt clusters formed by vapor deposition range from 20 to 50 Å. However, the heights of the Pt clusters formed by these two methods are similar and range from about 10 to 40 Å. Distribution of Pt cluster size was formed to be depended on impregnated platinum loading on an oxidized HOPG surface prepared by impregnation. The platinum clusters observed were not mobile on an oxidized HOPG surface during scanning, and STM images were very reproducible.