  The properties of thermal material coexisting with non-thermal emitting
plasma and strong magnetic field in the powerful jets of Active Galactic Nuclei
(AGN) are examined. Theoretical and observational constraints on the physical
properties of this `cold' component are determined. While the presence of a
thermal component occupying a fraction ~1.E-8 of the jet volume is possible, it
seems unlikely that such a component is capable of contributing significantly
to the total jet energy budget, since the thermal reprocessing signatures that
should appear in the spectra have not, as yet, been detected.
