Platinum vapor deposited at submonolayer coverages on the basal plane (bp) of highly oriented pyrolytic graphite (HOPG) in ultrahigh vacuum (UHV) was characterized in situ by X-ray photoelectron spectroscopy (XPS) and in air by scanning tunneling microscopy (STM). The effect of defect sites on the electronic and morphological nature of these deposits was investigated by first sputtering the HOPG(bp) with Ar+, while blocking a section of the specimen with a mask and then exposing the entire surface to Pt vapor. An analysis of the XPS spectra indicated that in the Ar+-sputtered region, the coverage, normalized by the Pt atom flux, was about 50% larger and the Pt binding energy ca. 0.2 eV higher, i.e. slightly more oxidized, than in the pristine region. STM images revealed discrete cluster-type features a few nm in average diameter and one to two atoms high, distributed fairly evenly within each region. The total amount of Pt contained within these clusters, as estimated from these images, however, could not account for more than 1-10% of that detected by XPS. The density of clusters was larger and their average size somewhat smaller in the sputtered, compared to the pristine regions, indicating that an increase in the density of defect sites promotes the growth of a larger number of clusters, albeit of smaller size.