  We discuss XMM-Newton and BeppoSAX observations of MKN 231, the
lowest-redshift Broad Absorption Line (BAL) QSO known so far and one of the
best-studied Ultra Luminous Infrared Galaxies. By combining the XMM-Newton
spectral resolution and the high-energy bandpass of BeppoSAX we have been able
to study in more detail than previously possible its 0.2--50 keV spectral
properties. The BeppoSAX PDS data unveiled, for the first time, a highly
absorbed ($N_H\sim 2\times 10^{24}$cm$^{-2}$) power-law component. We find
that: a) below 10 keV we are seeing only reprocessed radiation through
reflection and/or scattering; b) the intrinsic 2-10 keV luminosity of MKN 231
is $1^{+1.0}_{-0.5} \times 10^{44}$ ergs s$^{-1}$, i.e. more than an order of
magnitude greater than previous measurements; c) the starburst activity
significantly contributes to the soft ($E<2$ keV) X-ray emission.
