  Observing objects in transition from pre- to young Planetary Nebula (PN),
when the central star radiation starts to excite the envelope, can help us to
understand the evolution of the circumstellar ejecta and their shaping
mechanism/s. In our project we have selected a sample of hot post-AGB stars as
Transition Phase candidates. Radio observations have led to detect free-free
radiation from an ionized shell in about half of our targets, providing us with
two sub-samples of ionized and non ionized Transition Objects. We are now using
IRAC and IRS on the Spitzer Space Telescope to determine if extended emission
is present (IRAC) and to study our targets' chemistry (IRS). In particular, by
comparing spectra from the two sub-samples, the IRS observations will enable us
to check how the presence of an ionization front effects the circumstellar
envelope. The IRAC measurements, combined with previous ones in the literature,
will give us information on the extent and physical conditions of the dust
components.
