  The bright, strongly variable BL Lac object PKS 2155-304 was observed by XMM
for two essentially uninterrupted periods of ~11 and 16 hr on 30-31 May 2000.
The strongest variations occurred in the highest energy bands. After scaling
for this effect, the three softest bands (0.1-1.7 keV) showed strong
correlation with no measurable lag to reliable limits of $\tau \ls 0.3$ hr.
However, the hardest band (~3 keV) was less well-correlated with the other
three, especially on short time scales, showing deviations of ~10-20% in ~1 hr
although, again, no significant interband lag was detected. This result and
examination of previous ASCA and BeppoSAX cross-correlation functions suggest
that previous claims of soft lags on time scales of 0.3-4 hr could well be an
artifact of periodic interruptions due to Earth-occultation every 1.6 hr.
Previous determinations of the magnetic field/bulk Lorentz factor were
therefore premature, as these data provide only a lower limit of $B
\gamma^{1/3} \gs 2.5$ G. The hardest band encompasses the spectral region above
the high-energy break; its enhanced variability could be indicating that the
break energy of the synchrotron spectrum, and therefore of the underlying
electron energy distribution, changes independently of the lower energies.
