  Preliminary VLBA polarisation results on 6 ``blazars'' from 6.5-cm to 7-mm
are presented here. Observing at several different wavelengths, separated by
short and long intervals, enabled reliable information about the magnetic (B)
field structure to be obtained and for the effect of Faraday Rotation to be
determined and corrected. For all sources the magnitude of the core Rotation
Measure (RM) derived from the shorter wavelength data was greater than that
derived from the longer wavelength data, consistent with a higher electron
density and/or B-field strength closer to the central engine. A transverse RM
gradient was detected in the jet of 0954+658, providing evidence for the
presence of a helical B-field surrounding the jet. The RM in the core region of
2200+420 (BL Lac) displays sign changes in different wavelength intervals (on
different spatial scales); we suggest an explanation for this in terms of
modest bends in a helical B-field surrounding the jet.
