The mid-IR emission (12 and 25um IRAS bands) of the interstellar
 medium is produced by populations of small grains emitting during
 temperature fluctuations. In region of rather low density and
 strong radiation fields (close to hot stars), 12um emission
 contrasts can only be explained if small grains are destroyed
 and or processed by stellar photons.
 For the large molecule population (PAHs) of interstellar dust,
 two photodissociation mechanisms have been proposed. The detailed
 molecular processes involved are poorly known, however, and
 their efficiency in interstellar clouds unknown.
 On the other hand the same class of objects exhibits a high 25
 to 12um emission ratio that cannot be explained by varying the
 small grains abundances.
 At present there is no interpretation of this colour behaviour.
 To better understand the behaviour of small grains in strong
 stellar radiation fields, we propose to carry out mid-IR
 spectroscopic observations on low density (10 cm-3), strongly
 irradiated (more than 50 times the standard interstellar
 radiation field) regions, away from ionization fronts.
 A comparative analysis between spectra of regions where small
 grains photoprocessing is actively underway and regions where it
 is not, will provide important information on the grain property
 changes induced by photons and help identify the most abundant
 products of photodissociation. This latter point will constrain
 current ideas on the dominant photodestruction processes and allow
 a quantitative estimate of their efficiencies in the interstellar
 medium.