  We present new NICMOS and ACS observations of the quasar jet PKS 0637-752,
and we use them, together with existing multiwavelength observations, to
produce the most complete spectral coverage of the source to date. We explore
the implications of these observations in the context of models for the jet
X-ray emission. By relaxing the assumption of equipartition, we undertake an
exhaustive study of the parameter space for external Compton off the CMB
(EC/CMB) model. We find that the multiwavelength observations exclude a
magnetic field dominated jet. Using the method proposed by Georganopoulos et
al. (2005) for probing the jet matter content we show that protons are needed
for practically all jet configurations, extending a previous application of the
method by Uchiyama et al. (2005) that was based on exploring three particular
jet configurations. We also show that equipartition is the only configuration
that can reproduce the observations and have one proton per radiating lepton.
We finally present a rather model - independent argument that the jet has a
spine-sheath flow pattern, with the spine being faster and emitting most of the
IR-optical-X-ray emission.
