To control the size and structure of supported Pt catalysts, the influence of additional metal particles and the effect of supports were elucidated during the cracking reaction of n-dodecane under supercritical reaction conditions. The dynamical changes in nanocatalysts and catalytic activity are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed heating, in situ Small Angle X-ray Scattering (SAXS) and X-ray Absorption Near Edge Structure (XANES). In situ SAXS results indicate that the stability of the catalysts increases with Sn concentration. In situ XANES analysis reveals that the degree of oxidation and the electronic states of catalysts are dependent on the amount of Sn. Carbonaceous deposits over spent catalysts were characterized by Raman spectroscopy, indicating that the highest Sn loading inhibits the formation of disordered graphitic lattices, which leads to an increased catalytic activity. SiO₂, γ-Al₂O₃ and Mg(Al)O_x were employed as supports to investigate the support effect on the stability of Pt catalysts. In situ SAXS and XANES results clearly show the improved stability of catalysts on γ-Al₂O₃ and Mg(Al)O_x supports compared to Pt catalysts on SiO₂ and the electronic states of catalysts are strongly influenced by support materials.