Gold nanoparticles (GNPs) are stable in a number of linear polymer solutions, including poly(vinyl alcohol), poly(N-vinyl-2-pyrrolidone), hydroxyethyl cellulose, and poly(ethylene oxide) (PEO). Of these, PEO, with a wide range of molecular mass (from 0.3 to 8 MDa), is particularly attractive for exploring the interaction between the GNPs and polymer molecules. We have found that the colors of the GNPs are significantly different in PEO solutions at concentrations below and above entanglement threshold concentration (Phi*), which allows one to determine Phi* values for different sizes of PEO from the inflection points of the plots of the absorbance at 600 nm against the concentration of PEO. The Phi* values are close to those obtained by the viscosity measurements, showing the usefulness of this simple method. Transmission electron microscopy images have confirmed that the change in the absorbance is due to the aggregation and/or agglomeration of the GNPs in PEO solutions.