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.