<p>The knowledge of fundamental quantities of materials like the optical constants, refractive index (<i>n</i>) and extinction coefficient (<i>k</i>), is necessary for quantitative description of their optical response. In this work, the infrared optical constants of chitosan and cellulose nanocrystals were determined by Kramers–Kronig analysis of attenuated total reflection (ATR) spectra for polarization perpendicular to the plane of incidence. To achieve good contact between the sample and the ATR crystal, an acetic acid solution of chitosan and cellulose nanocrystal suspension were directly deposited on the ATR crystal and allowed to dry. The reliability of the Kramers–Kronig analysis was tested using deionized water as an example. The usual correction employed in the ATR method that includes the penetration depth in the low absorption limit shows large discrepancies in regions of strong anomalous dispersion in <i>n</i> with the absorption coefficient. A very good description is found when the effective thickness and the retrieved optical constants are considered. The anisotropy of optical constants of partially ordered birefringent dip-coated cellulose nanocrystal film is also investigated.</p>