Infrared spectra of dusty stellar envelopes reflect the abundances
 of dust grains and/or large molecules in the circumstellar material
 and the effects of the stellar radiation field. Prominent emission
 bands due to silicon carbide or silicate grains are commonly seen in
 cool giants, reflecting the carbon-rich or oxygen-rich chemistry of
 the circumstellar material. Carbon-rich dust heated by hot stars
 (> about 10000 K) often displays emission in the familiar UIR bands
 which resembles the emission from NGC 7027 or other C-rich planetary
 nebulae (although significant differences in relative intensities and
 band profiles are evident from source to source). In contrast some
 dusty envelopes around cooler stars (in the range 6000-8000 K) display
 peculiar emission spectra which appear to be related to the UIR
 emission bands, but which may arise from species that do not survive
 the harsher radiation fields of the hotter stars, and may therefore
 represent emission from much more fragile carbon clusters.

 We wish to investigate the detailed dust emission spectra of a
 sample of stars which have been found to have peculiar emission
 features from ground-based and/or IRAS LRS spectra. These consist of
 a group of 4 post-AGB stars, a Carbon star/A star binary system and
 an Ae/Be star exciting reflection nebulosity which appears to have a
 particularly pure UIR emission spectrum. The post-AGB stars seem to
 form a sequence of increasing departure from the `standard' UIR band
 spectrum and show a strong broad emission complex at 11-13 microns
 together with weaker features near 10.5 microns and a prominent
 emission peak near 21 microns. Ground-based spectra show evidence
 of substructure in the 21 micron band and we wish to follow this up
 with higher resolution and higher quality spectra with ISO. ISO
 provides the only way to sample the whole 2.4 - 45 micron region of
 these intriguing objects