  We investigate freeze--out in hydrodynamic models for relativistic heavy--ion
collisions. In particular, instantaneous freeze--out across a hypersurface of
constant temperature (``isothermal'' freeze--out) is compared with that across
a hypersurface at constant time in the center-of-momentum frame
(``isochronous'' freeze--out). For one--dimensional (longitudinal) expansion
the rapidity distributions are shown to differ significantly in the two
scenarios, while the transverse momentum spectra are remarkably similar. We
also investigate the rapidity distribution in greater detail and show that the
Gaussian-like shape of this distribution commonly associated with the Landau
expansion model in general emerges only if one neglects contributions from
time-like parts of the isothermal freeze--out hypersurface.
