A theory and technique for the measurements of the full state of polarization (SOP) for wideband electromagnetic HF emissions from the ionosphere are introduced. The technique was employed in a recent experiment at the Sura ionospheric HF pumping facility near Nizhniy Novgorod, Russia, to measure the full SOP of stimulated electromagnetic emissions (SEE) in their steady state. Measurements were made for O-mode polarized pump waves at two different frequencies: one below and one slightly above the fourth electron gyroharmonic frequency. The wideband spectral parameters measured in this experiment include the horizontal intensity, the degree of horizontal circular polarization, the degree of horizontal polarization and the tilt angle of the horizontal polarization ellipse. Evaluation of the data collected demonstrates that the technique can be used for HF polarimetry and that, in particular, such measurements provide useful new diagnostics for SEE. Among the most important results regarding the polarimetry of SEE were low degrees of circular polarization in the frequency range between the peak of the broad upshifted maximum and the pump; some SEE features had degree of circularity and tilt angle values that were consistent with slightly oblique waves indicative of a distributed SEE source region; and the pump was found to have changed from an initial left-handed polarized state to a right-handed polarized state during steady state. ¿ 2001 American Geophysical Union