We propose to observe the far-infrared lines of O, OH and H2O in
 Photon-Dominated Regions (PDRs) associated with Partially Ionized
 Globules (PIGs) in the HII region IC1396.
 The distances of the various globules to the illuminating O6
 star HD206267 vary by more than a factor of 4. Since the incident
 radiation field controls the formation and destruction of atomic and
 molecular species, the abundances of O, OH and H2O are expected to
 vary greatly from globule to globule and will provide stringent tests
 of the O/OH/H2O transition in PDR models.
 These far-infrared lines cannot be observed from the ground. The
 OH and H2O lines are expected to be weak, hence the high sensitivity
 of ISO is essential for this study.