In this paper a method is developed to describe the change of polarization of an electromagnetic wave traversing a non-uniform, magnetized plasma in terms of a 2*2 matrix. It is shown by model calculations that this transformation matrix yields the same results as those obtained by the well-established Poincare sphere method. The formal equivalence of the plasma matrix to Jones matrices representing polarization-sensitive optical components makes it possible to devise simple and versatile simulation programmes for real polarimetric instruments. As an example, the heterodyne polarimeter used on the TEXTOR Tokamak is briefly discussed.