  We have detected significant Rotation Measure variations for 9 bright
pulsars, as a function of pulse longitude. An additional sample of 10 pulsars
showed a rather constant RM with phase, yet a small degree of RM fluctuation is
visible in at least 3 of those cases. In all cases, we have found that the
rotation of the polarization position angle across our 1.4 GHz observing band
is consistent with the wavelength-squared law of interstellar Faraday Rotation.
We provide for the first time convincing evidence that RM variations across the
pulse are largely due to interstellar scattering, although we cannot exclude
that magnetospheric Faraday Rotation may still have a minor contribution;
alternative explanations of this phenomenon, like erroneous de-dispersion and
the presence of non-orthogonal polarization modes, are excluded. If the
observed, phase-resolved RM variations are common amongst pulsars, then many of
the previously measured pulsar RMs may be in error by as much as a few tens of
rad m-2.
