A detailed study of the polarization of the electron cyclotron radiation in Alcator has been undertaken. A polarizing Michelson interferometer and a Fabry-Perot and rotating polarizer combination are used to obtain frequency resolved measurements of the polarization parameters. The observed direction of polarization is consistent with the expected limiting polarization of the modes propagating out of the plasma, including the effects of poloidal field. In certain circumstances the observed polarization varies with frequency, equivalent to a slight shift of the extraordinary mode to lower frequencies. This is attributed to the toroidal magnetic field ripple.