  Molecular line observations of starless (prestellar) cores combined with a
chemical evolution modeling and radiative transfer calculations are a powerful
tool to study the earliest stages of star formation. However, conclusions drawn
from such a modeling may noticeably depend on the assumed thermal structure of
the cores. The assumption of isothermality, which may work well in
chemo-dynamical studies, becomes a critical factor in molecular line formation
simulations. We argue that even small temperature variations, which are likely
to exist in starless cores, can have a non-negligible effect on the
interpretation of molecular line data and derived core properties. In
particular, ``chemically pristine'' isothermal cores (low depletion) can have
centrally peaked C$^{18}$O and C$^{34}$S radial intensity profiles, while
having ring-like intensity distributions in models with a colder center and/or
warmer envelope assuming the same underlying chemical structure. Therefore,
derived molecular abundances based on oversimplified thermal models may lead to
a mis-interpretation of the line data.
