  Broad absorption line quasars (BALQSOs) are key objects for studying the
structure and emission/absorption properties of AGN. However, despite their
fundamental importance, the properties of BALQSOs are still not well
understood. In order to investigate the X-ray nature of these sources, as well
as the correlations between X-ray and rest-frame UV properties, we compile a
large sample of 88 BALQSOs observed by XMM-Newton. We performed a full X-ray
spectral analysis on a sample of 39 sources with higher X-ray spectral quality,
and an approximate HR analysis on the remaining sources. Using available
optical spectra, we calculate the BALnicity index and investigate the
dependence between this optical parameter and different X-ray properties.
  Using the neutral absorption model, we found that 36% of our BALQSOs have NH
< 5x10^21 cm^-2, lower than the expected X-ray absorption for such objects.
However, when we used a physically-motivated model for the X-ray absorption in
BALQSOs, i.e. ionized absorption, \sim 90% of the objects are absorbed. The
absorption properties also suggest that LoBALs may be physically different
objects from HiBALs. In addition, we report on a correlation between the
ionized absorption column density and BAL parameters. There is evidence (at 98%
level) that the amount of X-ray absorption is correlated with the strength of
high-ionization UV absorption. This correlation, not previously reported, can
be naturally understood in virtually all BALQSO models, as driven by the total
amount of gas mass flowing towards the observer.
