Since a decade, polycyclic aromatic hydrocarbons (PAHs) have been
 proposed to be an important dust component of the interstellar medium.
 This assumption is mostly supported by the comparison between the
 observational data in the IR and the laboratory spectra.  However,
 the exact chemical structure of the interstellar molecules is still
 poorly known and no individual species have been yet identified.
 Part of the difficulty is probably related to the chemical alteration
 of these compounds when submitted to the UV interstellar field. More
 specifically, the molecules can be ionized and photodissociated.
 Evidence of photochemical evolution of PAHs have been reported from
 ground-based studies in the 3um window for a few reflection nebulae
 and the Orion Bar photodissociation region.  These spatial studies
 show strong variations of the relative intensities of the 3.3um band
 and its satellite band at 3.4um.  Detailed photochemical models
 predict similar variations in the modes in the 5-8um region.
 ISO will provide the opportunity of studying the spatial distribution
 of the PAH bands at 5.2, 5.6, 6.2, 6.9 and 7.7um which measure C-H,
 CH3, triple-bonded CC, and C=C vibrational modes.  We propose to study
 these bands in a number of HII regions and  reflection nebulae.  Due
 to the geometrical dilution and dust extinction, the FUV intensity
 varies considerably over these nebulae.  Thus, the relative spatial
 distribution of these bands will reflect the varying chemical
 nature of the PAHs caused by the radiation field.