The degree of polarization is a central quantity in the characterization of random electromagnetic beams and it has been introduced both in the time and the frequency domains. Physically, the spectral degree of polarization corresponds to that of a field obtained by filtering a generally broad-band light by a narrow-band filter. It is known that the two degrees of polarization can generally assume different values and no simple relationship exists between them. For example, a stationary field can have an arbitrary degree of polarization in the frequency domain although the field is fully unpolarized in the time domain. Moreover, the field can be fully polarized at every frequency, but in the time domain the field may be anything between unpolarized and polarized. In this work, we study the connections between the time and frequency domain degrees of polarization. We introduce a mean spectral degree of polarization and show that it provides an upper limit for the value of the time-domain degree of polarization that can be obtained if arbitrary unitary transformations are performed in the frequency domain. A mean spectral degree of polarization equal to one indicates that the field is fully polarized in the frequency domain and thus can be made fully polarized also in the time domain by unitary transformations in the frequency domain, i.e., without absorption of energy.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.