Self-organized alloying of Au with Pt in nanoclusters on a thin film of Al2O3/NiAl(100) is active at 300 K, toward a preferred bimetallic structure. The photoelectron spectra of Au nanoclusters (diameter 1.0-3.0 nm and height 0.2-1.0 nm) formed from vapor deposition on thin-film Al2O3/NiAl(100) show that the binding energy (BE) of Au 4f7/2 decreases from 85.3 ± 0.1 eV upon incorporating subsequently deposited Pt, and saturates near 84.3 ± 0.1 eV with increasing Pt deposition; the line width decreases simultaneously from 1.9 ± 0.1 eV to a minimum near 1.1 eV. The same minimal BE and line width for Au 4f are observed on both deposition of the metals in the reverse order (first Pt and then Au) and with concurrent deposition of Au and Pt. The significant BE shift indicates increased screening, attributed on the basis of the density-functional calculations primarily to the increased coordination number of the alloying Au atom and secondarily to the charge transfer from Pt to Au; the narrowing of the spectral line reflects improved structural order of the clusters, which is corroborated by the diffraction measurements, and enhanced uniformity of the chemical environment around each Au atom. We argue accordingly that the alloying clusters form preferentially either an Au core-Pt shell structure or an Au-Pt intermixed structure with no Au exposed on the cluster surface; the bimetallic clusters have a fcc phase and grow with their facets either (111) or (001) parallel to the θ-Al2O3(100) surface.