The origin of the small scale color variations of the dust
 emission as revealed by the IRAS data in nearby interstellar
 clouds have remained a puzzle in spite of more than four
 years of dedicated efforts. One well settled result seems to
 be that these variations are due to local changes in the
 small dust particles properties (size distribution, optical
 properties). Spectral observations of the gas in a few
 bright mid-IR clouds (in the millimetric, visible and UV
 ranges) reveal that highly energetic processes are present
 in these clouds (excited transitions, broad lines, existence
 of molecules formed via endothermic reactions) although
 these clouds belong to the category of the "quiescent"
 ordinary interstellar clouds. The nature of the energetic
 processes at work in this component of the interstellar
 clouds is unknown. Magnetohydrodynamic shocks and
 intermittent dissipation of turbulence have been considered,
 but none of these processes is entirely satisfactory. Since
 we believe that this phenomenon is widespread in
 interstellar matter and may be of great significance for the
 evolution of dust particles and of that of the gas itself,
 we think that it deserves further investigation. The
 spectrometers aboard ISO are ideally suited for the spectral
 investigation of the properties of the highly excited gas,
 not only because the column densities of excited gas are
 small and therefore would escape detection by any less
 sensitive device, but also because the wavelength coverage
 of these instruments allows the observation of lines which
 will provide critical tests of the mechanism(s) which could
 be responsible for dust color variations.