We propose to follow up our recent discovery of the 3.4um
 interstellar absorption feature in the nuclear spectra of
 several type 2 Seyferts, by using ISO to search for the
 feature in higher redshift active galaxies such as narrow
 line radio galaxies and to probe the 5-9 um region for
 corresponding absorption features. The absorption almost
 certainly arises in the dense molecular torus surrounding
 the active core in type 2 Seyfert galaxies.  Detection
 of similar IR absorption features in narrow line radio
 galaxies would be strong evidence in favour of unified
 models of AGN in which both type 2 Seyferts and NLRG have
 an active core partially or wholly hidden behind a thick,
 dense torus of obscuring material.  The 3.4um feature can
 be remarkably deep in regions with very high columns of
 dust and probes a different temperature regime from CO
 molecular absorption features.  The proposed observations
 will provide details of the physical structure of the torus,
 and also improve our understanding of the formation of
 interstellar grains.