  We present three observations (four exposures) of Mkn 421 performed by
XMM--Newton in the Autumn of 2002, concentrating on the EPIC--PN camera data.
The X-ray spectra were soft and steepening toward high energies. The source was
highly variable and the hardness ratio plots displayed a clear
harder--when--stronger correlation. During two complete flares the source
showed strong spectral evolution: a hardness ratio and a time resolved spectral
analysis revealed both a clockwise and a counterclockwise rotating loop
patterns, suggesting the presence of temporal lags between different energy
bands variations. We confirmed this result and estimated the delay amounts with
a cross--correlation analysis performed on the single flares, discussing also
variability patterns that could reproduce the asymmetry seen in the
cross-correlation function. We verified our findings reproducing the two flares
with analytical models. We obtained consistent results: during one flare, Mkn
421 displayed soft lags, while in the other case it showed hard lags. In both
cases, the entity of the delays increases with the energy difference between
the compared light curves. Finally, we discussed the presence and the frequency
dependence of the temporal lags as an effect of particle acceleration, cooling
and escape timescales, showing that our data are consistent with this picture.
