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Optical properties of spin-cast chitosan films were determined in the vacuum ultraviolet (VUV) through visible regions of the spectrum using spectroscopic ellipsometry. The onset of absorption in the ultraviolet was determined for chitosan films modified with succinic anhydride (SA) and glycidyl phenyl ether (GPE). This absorption was accounted for by including Gaussian and Tauc–Lorentz shaped oscillators in the optical model. VUV through visible optical constants were determined for these modified films, as well as for immunoglobulin G (IgG) attached to GPE-modified samples. Several resonant oscillator structures exhibit greatly differing oscillator parameters for the as-deposited, as well as SA- and GPE modified films. These in-plane and out-of-plane oscillator strengths, energy positions, and broadening were determined and tabulated, and their chemical origins identified. A phenyl resonant π-bond at approximately 6.3 eV was observed for the GPE-modified sample. This was not present in the bare chitosan data. GPE contains a phenyl group in the molecule and absorption due to it is clearly identified. Clarity of data after surface chemical modification is due to the high surface sensitivity of VUV ellipsometry.