Soft x-ray photoelectron spectroscopy (SXPS) has been used to study ultrathin Pt films on silicon dioxide as model supported-catalyst materials. Using monochromatic synchrotron radiation. Pt 4f rind Si 2p core level photoelectron peaks were measured as a function of platinum coverage in the range 0-10 hit. The bulk silicon and silicon dioxide film Si 2p peaks each show a binding energy decrease within the first ML of dosing. However, the effect is stronger for the silicon dioxide Si 2p peak, indicating an increased screening of the 2p electrons by the metal overlayer. We also observe a monotonic increase of the work function of the ultrathin film Pt/SiO2 system with coverage from 4.52 initially to 5.58 eV at similar to 10 ML. The Pt 4f(7/2) core level binding energy decreased from similar to 72.2 to similar to 70.9 eV between 0 and similar to 10 ML coverage. This binding energy shift at low dose (less than or equal to 1 ML), the coverage dependence of the Pt line shape and intensity, and the large saturation coverage for the work function are each consistent with two-dimensional cluster-island growth. The metallic nature of the Pt overlayer with dose is quantitatively verified by nonlinear, least-squares numerical fitting of the Pt;tf SXPS peak line shapes with Gaussian-broadened Doniach-Sunjic functions.