Neutral pion production in central 158A GeV Pb-208 + Pb-208 collisions has
been studied in the WA98 experiment at the CERN Super Proton Synchrotron. T
he pi(0) transverse mass spectrum has been analyzed in terms of a thermal m
odel with hydrodynamic expansion. The high accuracy and large kinematic cov
erage of the measurement allow one to limit previously noted ambiguities in
the extracted freeze-out parameters. The results are shown to be sensitive
to the shape of the velocity distribution at freeze-out.