  We have considered all blazars observed in the X-ray band and for which the
slope of the X-ray spectrum is available. We have collected 421 spectra of 268
blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose
analysisis presented here. The X-ray spectra of blazars show trends as a
function of their power, confirming that the blazar overall energy distribution
can be parameterized on the basis of one parameter only, i.e. the bolometric
luminosity. This is confirmed by the relatively new hard (2-10 keV) X-ray data.
Our results confirm the idea that in low power objects the X-ray emission
mechanism is the synchrotron process, dominating both the soft and the hard
X-ray emissions. Low energy peaked BL Lac objects are intermediate, often
showing harder spectra in the hard X-ray band, suggesting that the synchrotron
process dominates in the soft band, with the inverse Compton process dominating
at high energies. The most powerful objects have X-ray spectra that are flat
both in the soft and in the hard band, consistent with a dominating inverse
Compton component.
