The properties of thermal material co-existing with non-thermal emitti
ng plasma and strong magnetic fields in the powerful jets of active ga
lactic nuclei (AGN) are examined. Theoretical and observational constr
aints on the physical properties of this 'cold' component are determin
ed. While the presence of a thermal component occupying a fraction sim
ilar to 10(-8) of the jet volume is possible, it seems unlikely that s
uch a component is capable of contributing significantly to the total
jet energy budget, since the thermal reprocessing signatures that shou
ld appear in the spectra have not, as yet, been detected.