The goal of this proposal is to determine the chemical properties of
 the dust shells of proto-planetary nebulae (PPN) through a study of
 their short-wavelength, and in a few cases their long-wavelength,
 infrared spectra.  Spectral features in the 10 to 20 micron region
 indicate the chemical nature (oxygen or carbon-rich), and the strengths
 of the features relate to the physical properties of the shells.
 A few bright carbon-rich PPN have been observed to show PAH
 features and an unidentified 21 micron emission feature.  We propose to
 observe a sample of IRAS sources which have the expected properties of
 PPN and for which we have confirmed a visible counterpart.  These we
 have observed extensively from the ground - visible photometry, near-
 infrared photometry, visible spectroscopy - and are well prepared to
 interpret the infrared spectra of these objects in the context of their
 overall properties.  This study will greatly advance our understanding
 of this important evolutionary stage.  Only ISO provides the spectral
 range and resolution necessary to carry out these observations.