The ISO LWS spectrometer offers a unique opportunity to directly
 investigate the properties of several OH infrared lines in absorption
 against compact HII regions. Using observations of several optically
 thick and thin IR lines connecting the low lying rotationnal levels of
 OH and 18OH, we  wish to analyze the excitation and abundance of the
 dense gas immediately  adjacent to a sample of compact HII regions
 selected according to their radio characteristics.