  I present high-frequency polarimetric radio observations of the radio galaxy
3C171, in which depolarization is associated with an extended emission-line
region. The radio hotspots, known to be depolarized at low radio frequencies,
become significantly polarized above (observer's-frame) frequencies of 15 GHz.
There is some evidence that some of the Faraday rotation structure associated
with the emission-line regions is being resolved at the highest resolutions
(0.1 arcsec, or 400 pc); however, the majority remains unresolved. Using the
new radio data and a simple model for the nature of the depolarizing screen, it
is possible to place some constraints on the nature of the medium responsible
for the depolarization. I argue that it is most likely that the depolarization
is due not to the emission-line material itself but to a second, less dense,
hot phase of the shocked ISM, and derive some limits on its density and
temperature.
