  It is shown that kinetic freeze-out in relativistic heavy-ion collisions
invariably entails a non-trivial dependence of the freeze-out temperature on
the collision centrality. The centrality independence of the chemical
freeze-out temperature observed in Au+Au collisions at RHIC is therefore
inconsistent with the hypothesis that hadron abundances decouple kinetically
from inelastic hadron-hadron interactions. On the other hand, it is consistent
with the hypothesis that chemical decoupling is driven by the quark-hadron
phase transition, and that the observed universal chemical freeze-out reflects
its critical temperature, independent of the dynamical state of the collision
fireball as it passes through the phase transition.
