We propose to observe the [FeVII] 7.81um and 9.51um transitions towards
the nuclei of three bright Seyfert galaxies. In conjunction with optical
lines of [FeVII] and the mid-infrared transitions of [NeV] they will provide
a new diagnostic of temperature, extinction, and density of the narrow
line region. This diagnostic is little dependent on Av, and is based
exclusively on species with an excitation potential near 100eV, thus
minimizing the risk of mixing diagnostics from distinct regions. An important
result unique to ISO will be a model-independent and extinction-insensitive
estimate of the NLR Fe/Ne ratio. Extensive SWS observations of other
mid-infrared transitions, in particular of [NeV], have or will be obtained
for two sources within our MPE guaranteed time programme on infrared
spectroscopy of bright galactic nuclei.