Deuterated potassium dihydrogen phosphate (DKDP) is used for third harmonic generation (frequency tripling) in high power laser applications [Ferroelectrics, 72, 1987, 397-441; Int. Mater. Rev. 47, 2002, 113-152]. DKDP is a uniaxial anisotropic crystal, which is highly transparent in the visible and infrared spectral ranges. This makes DKDP challenging but well suited for study by both transmission ellipsometry and more conventional reflection-mode ellipsometry. Mueller matrix ellipsometry measurements were required because the data included both anisotropic and depolarizing effects. A rotating analyzer ellipsometer, with computer-controlled Berek compensator after the input polarizer and before the sample, allowed measurement of 11 Mueller matrix elements in transmission mode. Spectroscopic transmission Mueller matrix measurements allow the index difference Deltan = n(o) - n(c) to be determined throughout the transparent spectral range along with the orientation of the optic axis. Reflection mode ellipsometry is used to extract the absolute index of the ordinary component over the spectral range 0.73 to 9 eV (138-1700 nm). Determination of one index component allows calculation of the other component since the difference n(o) - n(e) is known from the transmission Mueller matrix measurements. (C) 2004 Elsevier B.V. All rights reserved.