  We propose a method for estimating the composition, i.e. the relative amounts
of leptons and protons, of extragalactic jets which exhibit X-ray bright knots
in their kpc scale jets. The method relies on measuring, or setting upper
limits on, the component of the Cosmic Microwave Background (CMB) radiation
that is bulk-Comptonized by cold electrons in the relativistically flowing jet.
These measurements, along with modeling of the broadband knot emission that
constrain the bulk Lorentz factor of the jets, can yield estimates of the jet
power carried by protons and leptons. We provide an explicit calculation of the
spectrum of the bulk-Comptonized (BC) CMB component and apply these results to
PKS 0637--752 and 3C 273, two superluminal quasars with Chandra-detected large
scale jets. What makes these sources 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 jet knot, which guarantees that
most of the electrons are cold there, 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 to observe the BC scattered CMB emission, the
Spitzer angular resolution (~ 1''-3) is considerably smaller than the `bridges'
of these jets (~10''), making it possible to both measure and resolve this
emission.
