Thin films of chitosan, a biopolymer, have attracted attention as plausible candidates for several niche applications. Such uses would be better served by a deeper understanding of the physical characteristics of the films and their relationship to polymer confinement. Therefore, we have used spectroscopic ellipsometry to investigate the optical properties of chitosan thin films prepared by spin coating. Thin films of chitosan exhibited uniaxial anisotropy with a higher in-plane refractive index. This suggested that the polymer chains were preferentially oriented in the plane of the thin film. The optical anisotropy, defined as the difference between the in-plane and the outof plane refractive indices, increased upon decreasing the (dry) film thickness and the consequent increase in the one-dimensional confinement of the polymer chains. Upon exposure to solvent vapor, the film thickness increased and was accompanied by an exponential decrease in the optical anisotropy with increasing exposure time. The characteristic time of the exponential decrease was of the order of a few minutes and weakly increased with the dry film thickness. Deswelling experiments suggested that the preparation-induced residual stresses contributed negligibly to the observed optical anisotropy, which, in addition to the initial (dry) film thickness, also depended on the weight fraction of the penetrant in the swollen film. The continued increase in the film thickness, however, indicated that the swelling progressed for several hours before reaching saturation. Assuming the existence of a compact, bound layer of chitosan that could not swell upon exposure to solvent vapor, we defined a time-dependent "effective swelling ratio". The swelling kinetics of chitosan thin films of different initial thicknesses could be described using this single function, which suggested that the swelling process in these films was qualitatively identical.