X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF) were combined in situ to study the ALD-based synthesis of Pt catalysts. This first time combination of synchrotron-based techniques was applied during the (methylcyclopentadienyl)trimethylplatinum/ozone deposition process executed at 150 degrees C on a silica support. A nucleation delay indicative for nanoparticle formation was observed for Pt loadings below 1 equivalent monolayer. XAS and XRF were recorded simultaneously at different catalyst loadings in this nucleation regime. Analysis of the combined in situ data yielded a quantitative picture of the evolution of the diameter, shape, lattice packing and density of the deposited Pt clusters. Additionally, the degree of oxidation at the cluster surface after the ozone pulse could be monitored. At the early start of the deposition process, Pt adatoms cluster together to form stable nuclei. A strong increase in the density of nuclei is seen below 0.16 equivalent monolayers, after which coalescence gradually occurs. From 0.04 to 0.71 equivalent monolayers, Pt clusters are fcc packed and correspond best to a hemispherical (1 1 1)-truncated cuboctahedral shape. By crosslinking the XRF and XAS data, a linear increase in cluster diameter with Pt loading is observed within this range. The surface of the Pt clusters is shown to be oxidized immediately after the ozone exposure. The degree of surface oxidation remains approximately constant for clusters with a 1-3 nm diameter. This surface oxygen is shown to be crucial for further growth during deposition. The combined application of in situ XRF and XAS thus allowed for an advanced identification of the ALD-deposited Pt nanoparticles. (C) 2014 Elsevier B.V. All rights reserved.