Defect-free cyclic poly(ethylene glycol) (-PEG) has been previously reported to be physisorbed onto gold nanoparticles (AuNPs) to drastically enhance the dispersion stability. The present study investigates the distinct behaviors of the physisorption of -PEG and the chemisorption of linear thiol-terminated PEG (HS-PEG-OMe) onto gold substrates and AuNPs. Thermogravimetric analysis (TGA) and quartz crystal microbalance (QCM) were used to quantify the adsorption and characterize the PEG layers, finding that -PEG exhibits lower adsorption density and the adsorption mass and energy dissipation of the layer are mostly independent of the molecular weight, unlike HS-PEG-OMe. Furthermore, dynamic light scattering (DLS) and small-angle neutron scattering (SANS) assessed the complexes under varying temperatures, demonstrating that AuNPs modified with -PEG maintain a consistent structure, whereas AuNPs with HS-PEG-OMe experience significant structural shrinkage as the temperature increases. The results suggest that -PEG interacts with the gold surface at multiple points, leading to superior structural stability compared to that of the single-point attachment of HS-PEG-OMe, which is more prone to dehydration.