  We propose a method for estimating the matter content of quasar jets which
exhibit Chandra - detected knots in their kpc scale jets. The method relies on
measuring the component of the Cosmic Microwave Background (CMB) radiation that
is bulk-Comptonized (BC) by the cold electrons in the relativistically flowing
jet. We apply our method to PKS 0637 - 752, a superluminal quasar with an one
-- sided Chandra - detected large scale jet. What makes this source
particularly suited for such a procedure is the absence of significant
non-thermal jet emission in the 'bridge', the region between the core and the
first bright knot, guaranteeing that most of the electrons in the bridge are
cold, and leaving the BC scattered CMB radiation as the only significant source
of photons in this region. At lambda=3.6-8.0 microns, the most likely band for
the BC scattered emission to appear, the angular resolution of Spitzer (~ 1''-
3'') is considerably smaller than the bridge of PKS 0637 -- 752 (~8''), making
it possible to both measure and resolve this emission.
