The object of ellipsometry is to determine the complex ratio ρ (ρ=tanifi exp(i&) ) of the reflected or transmitted polarization state parameters. Ellipsometry has traditionally been used as a tool for measuring the optical constants (n,k) of a surface, or when given the optical constants of the substrate, to determine some property such as the thickness of a thin film. The operation of a Photo Elastic Modulator (PEM) type ellipsometer is described in Mueller matrix terms. The phase shift (6) between orthogonal polarization components and the relative amplitude ratio (tan b) is contained in the Mueller matrix elements and can be obtained via combinations of measlired matrix elements using no mechanical motion. Polarization modulation ellipsometry offers a fast automatic means for obtaining the Mueller matrix for an optical component or system. A computer controlled implementation is described along with simple algorithms to extract the ellipsometric constants. Experimental results for a low phase shift "standard" reflector are given.