  This paper deals with the heating and the ionization of the interstellar
medium (ISM) in the 500 central pc of the Milky Way (hereafter Galactic center,
GC). We review the results of the Infrared Space Observatory (ISO) observations
of a sample of GC molecular clouds located far from thermal radiocontinuum or
far-infrared sources. For the first time, we have been able to study in detail
the dust continuum spectra from 40 to 190 microns founding a warm (30-40 K)
dust component in addition to the well known 15-20 K component. Fine-structure
lines observations have revealed the presence of diffuse ionized gas associated
with the molecular clouds. The effective temperature of the ionizing radiation
is higher than 33000 K. ISO has also allow us to measure the fraction of warm
(~ 150 K) H2 in the GC clouds, which is on average of 30 %. The observations of
the warm (a few 100 K) neutral gas are compatible with a Photon Dominated
Region (PDR) scenario.
