A strong emission feature at 21 microns was discovered by Kwok et al. (1989),
in the spectra of five carbon-rich proto-planetary nebulae (PPNe), using
spectra in the IRAS Low Resolution Spectrometer data base. Since then, the
reality of the feature has been confirmed by airborne (Omont et al. 1995a,
1995b) and ground-based observations (Kwok et al. 1993, Justtanont et al.
1996). Meanwhile more PPNe have been found to have a 21 micron feature in
their spectra (Kwok et al. 1995, Justtanont et al. 1996) making this band a
distinct signature of this class of objects.  We have recently identified 12
more candidates displaying the 21 micron feature, possibly including a few
young stellar objects and HII regions. However, the quality of the IRAS data
is not sufficient to prove the reality of the feature (Henning et al. 1996).
Furthermore, neither the ground-based data nor the original detections are
sufficient to identify the carrier of this feature. We propose to obtain ISO
SWS spectra of these sources in order to better constrain the carriers of
the 21 micron emission band and to hopefully identify them using existing
and future laboratory work.