  The possibility to detect simultaneously in the X-ray band the synchrotron
and Inverse Compton (IC)emission of intermediate BL Lac objects offers the
unique opportunity to study contemporaneously the low- and high-energy tails of
the electron distribution in the jets of these sources. We attempted to
disentangle the X-ray spectral variability properties of both the low- and
high-energy ends of the synchrotron and Inverse Compton emission of the
intermediate BL Lac object S5 0716+71. We carried out spectral, temporal and
cross-correlation analyses of the data from a long XMM-Newton pointing of S5
0716+71 and we compared our findings with previous results from past X-ray
observations. Strong variability was detected during the XMM exposure.Both the
synchrotron and Inverse Compton components were found to vary on time scales of
hours, implying a size of the emitting region of $R\la 0.7\delta /(1+z)$
light-hours. The synchrotron emission was discovered to become dominant during
episodes of flaring activity, following a harder-when-brighter trend. Tight
correlations were observed between variations in different energy bands. Upper
limits on time lags between the soft and hard X-ray light curves are of the
order of a few hundred seconds.
