  In this paper we analyse five observations of the BL Lac object AO 0235+16
performed with the Chandra and XMM-Newton satellites during the years
2000-2005. In the February 2002 observation the source is found in a bright
state and presents a steep X-ray spectrum, while in all the other epochs it is
faint and the spectrum is hard. The soft X-ray spectrum appears to be strongly
absorbed, likely by the intervening system at z=0.524, which also absorbs the
optical-UV radiation. We find that models that consider spectral curvature are
superior to single power law ones in fitting the X-ray spectrum. In particular,
we favour a double power law model, which agrees with the assumption of a
superposition of two different components in the X-ray domain. Both in the
Chandra and in one of the XMM-Newton observations, a tentative detection of the
redshifted Fe Kalpha emission line may suggest its origin from the inner part
of an accretion disc. Thermal emission from this accretion disc might explain
the UV-soft-X-ray bump that appears in the spectral energy distributions, when
the X-ray spectra are complemented with the optical-UV data from the Optical
Monitor onboard XMM-Newton. More likely, the bump can be interpreted in terms
of an additional synchrotron component emitted from an inner region of the jet
with respect to that where the lower-energy emission comes from. An inspection
of the X-ray light curves reveals that intraday variability occurs only when
the source is in a bright state.
