We plan to measure IR coronal lines in a sample of AGN with exceptionally strong forbidden high-ionization lines in their visual spectra. Observations of coronal lines permit the derivation of the flux density of energetic photons if the lines are photo-ionized. Hence these lines allow to measure the photon flux at energies of up to a few hundred eV, which cannot be measured directly because of (extragalactic and galactic) absorption of the UV and soft X-ray radiation. Therefore, such lines provide an efficient calorimeter for photons in the far-UV regime. The luminosity at energies of the order 100 eV can be probed by sampling lines of different ionization potentials. Since the strongest forbidden high-ionization lines (FHILs) are all concentrated in the near- and mid-IR (while the visual and groundbased IR ranges contain relatively weak FHILs only) ISO observations can provide particularly important information on this subject. In detail we wish to observe a set of objects which (according to a survey carried out by our group) shows exceptionally strong coronal lines in their visual spectra. Our objects furthermore cover a range of line widths which may reflect a range of the central mass and specific orientation. We will combine our IR results with our groundbased data to derive possible dust obscuration effects and corrections and we will compare our combined data with existing model computations to derive the radiation field causing the FHILs. Morover, we plan to use the line profile information from the IR spectra to obtain further constraints on the location and geometry of the coronal line emitting regions within the AGN complex.