  We present a detailed chemical analysis on the basis of high-resolution, high
signal-to-noise optical spectra of two post-AGB objects IRAS 22223+4327 and
IRAS 04296+3429. Both display the unidentified $21 \mu m$ feature in their
IR-spectra. The spectroscopic indicators provide accurate atmospheric
parameters of $T_{eff}$=6500 K, $log g=1.0$ and $\xi_t = 5.5 km/s$ for IRAS
2223+4327 and $T_{eff}$=7000 K, $log g=1.0$ and $\xi_t = 4.0 km/s$ for IRAS
04296+3429. Both photospheres are found to be metal-deficient with [Fe/H]= -0.4
and -0.7 respectively. C and N are found to be overabundant. The mean abundance
of all the measured s-process-elements is [s/Fe]=+1.0 for IRAS 2223+4327 and
+1.4 for IRAS 04296+3429. The distribution of the s-process elements can best
be described as due to a distribution of neutron exposures with a low mean
neutron exposure of $\tau_{0} = 0.2 mbarn^{-1}$. The 21 $\mu$m stars form an
interesting sub-group in the total post-AGB sample of stars, not only for their
IR characteristics, but also in a broader context of stellar (chemical)
evolution theory. They show, in contrast to other post-AGB stars, that the 3rd
dredge-up has been efficient during their AGB evolution. The mean neutron
exposure is lower than expected for their metallicity.
