This work introduces two new methodologies, gold Maximum Plasmon Peak Shifts (MaPPS) and Charge Transfer Relative Intensity Change (CT-RICh), for correlating the in situ UV-Visible-NIR response of supported metal nanoparticles with particle size (D p ) and adsorption site location. Gold MaPPS measures surface plasmon resonance peak shifts during alternating exposure to oxidizing (O 2 ) and reducing (H 2 ) species. These shifts serve as proxies for relative charge transfer (ΔN/N), derived from a Drude-Lorentz’s free electron model. By integrating ΔN/N with geometric site statistics (e.g., truncated octahedron, cuboctahedron, and icosahedron), MaPPS enables the estimation of dominant adsorption sites and particle sizes under reaction conditions. Complementarily, CT-RICh is proposed for non-plasmonic materials or those with weak plasmonic responses. It correlates redox-induced absorbance changes in the d-d transition region, arising from charge transfer to/from the support or the formation of oxidized metal species, with particle size and coordination. MaPPS and CT-RICh were demonstrated on Au/TiO 2 , Au/ZnO, Au/SrTiO 3 , and Au/SiO 2 systems with 3-9 nm Au particle sizes at 125 and 240 °C. The results confirm the preferential adsorption at undercoordinated Au sites (CN = 5, 6, and 7), primarily at the metal-support interface. At elevated temperatures, adsorption becomes increasingly restricted to corner sites (CN = 5) in smaller nanoparticles. Au MaPPS and CT-RICh Au particle size estimations based on a metal-support perimeter adsorption model showed high agreement with Au particle sizes obtained from TEM measurements. Deviations from the model were attributed to morphological dynamics, such as particles becoming increasingly oblate at larger particle sizes. Overall, MaPPS and CT-RICh provide a robust, simple in situ UV-visible characterization of adsorption site location and particle size for catalytic systems involving redox charge transfer. • New MaPPS and CT-RICh methods link UV-Vis data to Au nanoparticle size and sites • Undercoordinated (e.g., perimeter) sites (CN 5-7) identified as adsorption zones • Perimeter-based adsorption models enable accurate in situ particle size estimation • MaPPS accuracy validated by size prediction from relative plasmon peak shifts • CT-RICh with potential to probe charge transfer in redox-active catalysts