===================================================================== ==> In this proposal, more time is being requested for TMONTMER.1103FEAT ===================================================================== We propose to observe the interaction of X-rays from young stars in molecular clouds with the surrounding dust. For T Tauri stars in the outer regions of molecular clouds, the signature of this interaction is predicted to be the disappearance of PAH features within 0.01 to 0.1 pc from the star. For Herbig Ae/Be stars, which are usually surrounded by dense circumstellar dust, the signature of X-ray interaction may be the disappearance of PAH emission in the close vicinity of the star. From recent X-ray data provided by the ROSAT and ASCA satellites, we select 4 T Tauri stars in the Rho Oph cloud, and 6 Herbig Ae/Be stars with a wide range of X-ray luminosities. We also select 2 X-ray emitting dense cores associated with the Monoceros and Rosette molecular clouds. We use the imaging properties of ISOCAM (using 5 to 8 filters centered on UIB and silicate features and their references) to study the dependence of the UIB emission as a function of distance to the X-ray source. The results should constrain both our knowledge of the composition of the smallest dust grains, and of the heating of dust by hard radiation. The present proposal is a follow-up of a previously accepted proposal (PROPID = 1103FEAT, Priority 1), which has been done using 3 filters around 6 mum (LW4 to LW6). A preliminary analysis of the data, recently made available to us, shows that flux ratios obtained with these filters are indeed tracers of irradiation effects, justifying the present request for more observations at other wavelengths.