In order to examine cluster size dependence, mass-selected platinum clusters, Pt-n (n=2-5, 7, 8, 10, 15), were deposited on TiO2(110) surfaces at room temperature under soft landing conditions, and their core levels were investigated by x-ray photoelectron spectroscopy. Pt core-level shifts with cluster size are observed. The binding energies of Pt 4f(7/2) decrease steeply with increasing cluster size up to n=7 for Pt-n and decrease gradually for n >= 8. This inflection point (n=8) agrees well with the cluster size at a geometric transition (planar-to-three-dimensional) seen with scanning tunneling microscopy (STM) [N. Isomura, X. Wu, and Y. Watanabe, J. Chem. Phys. 131, 164707 (2009)]. It was found that the core-level shifts of mass-selected Pt clusters deposited on TiO2 are closely correlated with cluster geometries determined directly by atomic-resolution STM imaging. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3467033]