A shift in plasmon surface resonance is measured for colloidal solutions of gold nanoparticles upon adsorption of polyampholyte (gelatin). It is demonstrated that the shift in wavelength of the absorption maximum can quantitatively yield measurements of the adsorbed amount as well as information about the structure of the adsorbed polymer layer. Semiquantitative agreement is found with calculations based on the Mie theory in the quasi-static Limit. The method is demonstrated to provide quantitative measurements of the kinetics of biopolymer adsorption onto colloidal nanoparticles in solution.