We propose to observe a sample of high excitation molecular
 lines in a strip crossing the photodissociation regions (PDR)
 associated with NGC 7023, and NGC 2023. Since most of the
 hyperfine atomic lines, and high excitation molecular lines
 cannot be observed from ground based observatories, the chemical
 composition and physical conditions of these regions are far
 from being understood. The Central Programme includes
 observations of atomic and H2 rotational lines towards these
 reflection nebulae. But the rotational lines of molecules like
 OH, CH, CH2 or NH3 are not included at all. These lines are
 expected to be strong enough to be detected with ISO in a
 reasonable period of time, and the observation of these molecular
 lines is essential: 1) to estimate the abundance of these light
 molecules in the hot gas that constitutes the interface between
 the atomic and the molecular phases, 2) to determine the
 physical conditions of this hot molecular gas, and then 3)
 to understand the cooling mechanisms in these PDRs.