A method for calibrating an ellipsometer, and an ellipsometer including a processor programmed to control the analyzer, polarizer, and other ellipsometer components, and to process the data measured by the ellipsometer to perform the calibration method automatically. Where the ellipsometer's polarizer rotates and the analyzer remains fixed during measurement, the method determines coarse approximations of values A.sub.0 and P.sub.0, and then processes reflectivity data obtained at two or more analyzer angles to determine refined approximations of the values A.sub.0 and P.sub.0, where P.sub.0 is the angle of the polarizer's optical axis at an initial time, and A.sub.0 is the offset of the actual orientation angle of the analyzer from a nominal analyzer angle. Preferably the ellipsometer is a spectroscopic ellipsometer, the reflectivity data determine a tan.psi. spectrum and a cos.DELTA. spectrum for each of the analyzer angles, and the coarse approximations of A.sub.0 and P.sub.0 are refined by processing the reflectivity data by performing regression on A.sub.0 and P.sub.0 until the differences among the tan.psi. and cos.DELTA. spectra for several analyzer angles are minimized. Where the ellipsometer's analyzer rotates and the polarizer remains fixed during measurement, the method coarsely determines values A'.sub.0 and P'.sub.0, and then processes reflectivity data obtained at two or more polarizer angles to determine refined approximations of the values A'.sub.0 and P'.sub.0, where P'.sub.0 is the angle of the analyzer's optical axis at an initial time, and A'.sub.0 is the offset of the actual orientation angle of the polarizer from a nominal polarizer angle.