We propose a near to far-infrared spectroscopic survey of a set of very young planetary nebulae (PNe) selected on the basis of their very low-excitation characteristics, as revealed by their very low [OIII]5007 Ang. to H-alpha flux ratios. This survey will allow a study of the far-infrared emission properties of nebular dust grains, and the line-emission due to atomic/ionic species such as [CII] and [OI] in these objects. It is crucial to understand the properties of the dust (composition, amount) since these objects, as a class, are a major contributor of refractory dust grains to the interstellar medium (ISM). Our study will reveal the wavelength- dependence of the dust emissivity, and may lead to the discovery of new emission features which may help in determining the grain composition fully. The measurements of the atomic and ionic line fluxes will allow a determination of the physical characteristics of the warm gas present in the photodissociation regions (PDR) in these young PNe. We will therefore be able to determine the mass of gas and mass-loss rate characterising the PDRs in these PNe. Full LWS (43-197 5m) and SWS (2.4-45 5m) medium-resolution grating spectra will be obtained, using the LWS01 & SWS01 AOT's. These spectra will provide a comprehensive data-set for comparison with similar spectra being obtained for other classes of post-AGB and AGB objects (red giants, protoplanetary nebulae, evolved PNe) in the GTO program. Such a comparison is necessary in order to understand the evolution of the circumstellar matter during post-AGB evolution. This ISO study will also provide a necessary complement to our current studies of these "nascent" PNe using the HST, and ground-based mm/ submm-wave facilities. Close scientific collaboration with related GTP teams will be sought to conduct the most reliable and extensive analysis possible of the data obtained.