We propose to observe the far-infrared cooling lines of CII (158 mu),
 OI (63 and 145 mu), H2 S(1) (17 mu), S(2) (12 mu), and Si II (35 mu)
 in Photon-Dominated Regions (PDRs) associated with reflection nebulae
 exposed to radiation from cool stars with T_eff=5,000-22,000 K.
 Compared with classical PDRs illuminated by early O and B stars,
 these clouds close to A and F stars receive much less far-ultraviolet
 radiation. Since the incident radiation field controls both the
 heating and the formation and destruction of atomic and molecular
 species, the observations will allow stringent tests of PDR models
 in a regime that has so far been unexplored. In particular, the
 relative contribution of the various types of grains (including polycyclic
 aromatic hydrocarbons (PAHs)) to the heating can be investigated since the
 absorption cross section of PAHs has a different wavelength dependence from
 that of large grains. The dominant cooling lines of PDRs occur in the
 far-infrared and hence cannot be observed from the ground.
 In general, the expected line strengths for the reflection
 nebulae in our sample are 1-2 orders of magnitude less than those
 observed in these classical PDRs. Hence, the high sensitivity of ISO
 is essential for this study.