  The VLBI observations at 22 GHz and 43 GHz of the quasar 3C273 obtained
during a multi-frequency campaign in late 1992 in the radio, millimetre and
X-ray bands allow us to derive the components' angular sizes, their peak fluxes
and turnover frequencies. Lower limits to the Doppler factors have been derived
by comparing the observed X-ray fluxes with those predicted by the Synchrotron
Self-Compton model. Independent estimates of the Doppler factors were obtained
through the assumption of the energy equipartition between the particles and
the magnetic field. Of the five components used to model the first two
milli-arcseconds of the jet, apart from the core, two components are in
equipartition and the remaining two, at larger distances from the core, have
large Doppler factors and are mainly responsible for the X-ray emission due to
the Synchrotron Self-Compton process.
