Depolarization of optical components degrades the performance of a polarimeter, especially in the short wavelength region. Expressions for the light intensity in a null ellipsometer (NE) and a polarizer-sample-analyzer (PSA) system with depolarization errors in all components are given. Errors in Psi and Delta using IVE are mainly caused by depolarization of the compensator. A sample consisting of a thin sapphire slab 0.8 mm thick was tested. Delta measured by NE were shown to be accurate even for sample depolarization as large as 0.3, while those measured by rotating-analyzer ellipsometry (RAE) were seriously affected by sample depolarization. Methods for measuring depolarization were devised. The measured depolarization increases with the slit-width of the monochromator and the aperture size of irises and decreases with increasing wavelength. Depolarization spectra measured using NE, RAE and transmittance agree with one another.