  We report VLBA polarimetric observations of the CSS sources 3C119, 3C318, and
3C343 at 5 and 8.4 GHz. The CSS source 3C119 has source rest-frame RM values up
to ~10200 rad/m**2 in a region which coincides with a change in the direction
of the inner jet. This component is located ~325 pc from the core, which is
variable and has a peaked radio spectrum. In the case of 3C318, a rest-frame RM
of ~3030 rad/m**2 has been estimated for the brightest component which
contributes almost all of the polarised emission. Further, two more extended
components have been detected, clearly showing "wiggles" in the jet towards the
southern side of the source. The CSS source 3C343 contains two peaks of
emission and a curved jet embedded in more diffuse emission. It exhibits
complex field directions near the emission peaks, which indicate rest-frame RM
values in excess of ~6000 rad/m**2. The locations of the cores in 3C318 and
3C343 are not clear. The available data on mas-scale rest-frame RM estimates
for CSS sources show that these have a wide range of values extending up to
~40000 rad/m**2 in the central region of OQ172, and could be located at
projected distances from the core of up to ~1600 pc, as in 3C43 where this
feature has a rest-frame RM of ~14000 rad/m**2. RM estimates for cores in
core-dominated radio sources indicate that in addition to responding to an
overall density gradient of the magneto-ionic medium, geometry, orientation and
modes of fuelling may also play a significant role. In addition to these
effects, the high values of RM in CSS sources are possibly due to dense clouds
of gas interacting with the radio jets. The observed distortions in the radio
structures of many CSS sources are consistent with this interpretation.
