  We have observed a small sample of hot post-AGB stars with the InfraRed Array
Camera (IRAC) and the InfraRed Spectrograph (IRS) on-board the Spitzer Space
Telescope. The stars were selected from the literature on the basis of their
far-Infrared excess (i.e., post-AGB candidates) and B spectral type (i.e.,
close to the ionization of the envelope). The combination of our IRAC
observations with 2MASS and IRAS catalog data, along with previous radio
observations in the cm range (where available) allowed us to model the SEDs of
our targets and find that in almost all of them at least two shells of dust at
different temperatures must be present, the hot dust component ranging up to
1000 K. In several targets grains larger than 1 micron are needed to match the
far-IR data points. In particular, in IRAS 17423-1755 grains up to 100 micron
must be introduced to match the emission in the mm range.
  We obtained IRS spectra to identify the chemistry of the envelopes and found
that more than 1/3 of the sources in our sample have mixed chemistry, showing
both mid-IR bands attributed to Polycyclic Aromatic Hydrocarbons (PAH) and
silicate features. The analysis of the PAH features indicates that these
molecules are located in the outflows, far away from the central stars. We
consider the larger than expected percentage of mixed-chemistry targets as a
selection bias towards stars with a disk or torus around them. Our results
strengthen the current picture of mixed chemistry being due to the spatial
segregation of different dust populations in the envelopes.
