  The Infrared Space Observatory (ISO) has shown the complexity of the Galactic
center (GC) Interstellar medium (ISM) detecting, not only large column
densities of warm molecular gas (H2), but the emission of neutral atoms and
ions of low ionization potential (CII, OI, SiII,...) that should arise in
shocked or photon-dominated regions (PDRs). In addition, ISO has also detected
emission from ions like SIII, NeII, ArII, or NII (in some clouds we have even
detected NeIII and OIII) that should arise from HII regions that were
previously unsuspected due to the non-detection of Hydrogen recombination
lines. Here we review some ISO results on the large scale study of the GC ISM
and in particular, on the heating mechanisms of the clouds. Although, shocks
should play an important role on the physics and chemistry of the GC ISM, ISO
shows that the effect of radiation on the heating of the gas cannot be ruled
out with the simple argument that the dust temperature is lower than that of
the gas.
