We propose to obtain full range spectra (SWS and LWS) of a well defined sample of heavily obscured post-AGB stars recently discovered during an extensive multi-wavelength survey of bright IRAS sources with far-IR colors similar to those of PNe. They have recently left the AGB phase and are surrounded by thick circumstellar shells, showing no optical counterpart (or extremely faint) in the visible range and emitting most of their energy in the far-IR. Most of them are non-variable OH/IR stars, showing OH maser emission at 1612 MHz, but with a low IRAS variability index, indicating that the thermal pulses and the strong mass loss suffered by these stars at the end of the AGB have recently stopped and they are now post-AGB stars on their way to become new PNe. In a few cases, radio continuum emission has also been detected, indicating that the ionization of the envelope has already started even although nothing is yet visible in the optical range. We want to determine their global energy distribution to reconstruct their mass loss history and study the properties of the dust. This will be used to test the most recent models of AGB and post-AGB evolution which suggest that there is a correlation between the mass and age of these stars and their evolutionary tracks on the IRAS two-color diagram. ISO is the only instrument capable to obtain information about these stars as nothing can be extracted from observations in other wavelength ranges. Moreover, the targets included in this proposal are among the brightest infrared emitters between 2.4 and 197 microns, and SWS and LWS spectra will completely cover this spectral range.