An intensive study of the IRAS photometry of a wide sample of
 PNe and PPNe has shown that a clear correlation exists between the
 IRAS colors and the mass and age of PNe and PPNe. However, this
 correlation is affected in many cases by the presence of strong
 emission lines and a detailed analysis of the far infrared energy
 distribution is needed to model their evolutionary tracks
 through the IRAS two-color diagram.
  We propose to obtain full range spectra (SWS and LWS) of a well
 defined sample of PNe and PPNe in order to determine the
 contribution of these emission lines to the original IRAS fluxes.
  The analysis of the emission lines will be used to derive the
 chemical abundances and physical conditions of those PNe showing
 strong features, while the underlying continuum, basically due to
 dust emission, will be used to reconstruct their mass loss
 history and study the properties of the dust as a function of the
 position of the source in the IRAS two-color diagram, the mass of
 the progenitor star and evolutionary stage.