  A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2
with the VLA and the VLBA is presented. The relative astrometry in the VLBA
observations was precise on a level of a few microarcseconds. Spectral and
spatial evolution of the source are closely linked and these observations
allowed us to study in great detail a textbook example of a synchrotron
self-absorbed jet. We observe a phase where the jet gets frustrated, without
expansion and no spectral evolution. Then the jet breaks free and starts to
expand with apparent superluminal motion. This expansion is accompanied by a
strong spectral evolution. The results are a good confirmation of synchrotron
theory and equipartition for jets.
