  Asymptotic Giant Branch (AGB) stars are typified by strong dust-driven,
molecular outflows. For long, it was believed that the molecular setup of the
circumstellar envelope of AGB stars is primarily determined by the atmospheric
C/O ratio. However, recent observations of molecules such as HCN, SiO, and SO
reveal gas-phase abundances higher than predicted by thermodynamic equilibrium
(TE) models. UV-photon initiated dissociation in the outer envelope or
non-equilibrium formation by the effect of shocks in the inner envelope may be
the origin of the anomolous abundances. We aim at detecting (i.) a group of
`parent' molecules (CO, SiO, HCN, CS), predicted by the non-equilibrium study
of Cherchneff (2006) to form with almost constant abundances independent of the
C/O ratio and the stellar evolutionary stage on the Asymptotic Giant Branch
(AGB), and (ii.) few molecules, such as SiS and SO,which are sensitive to the
O- or C-rich nature of the star. Several low and high excitation rotational
transitions of key molecules are observed at mm and sub-mm wavelengths with
JCMT and APEX in four AGB stars: the oxygen-rich Mira WX Psc, the S star W Aql,
and the two carbon stars V Cyg and II Lup. A critical density analysis is
performed to determine the formation region of the high-excitation molecular
lines. We detect the four `parent' molecules in all four objects, implying
that, indeed, these chemical species form whatever the stage of evolution on
the AGB. High-excitation lines of SiS are also detected in three stars with
APEX, whereas SO is only detected in the oxygen-rich star WX Psc. This is the
first multi-molecular observational proof that periodically shocked layers
above the photosphere of AGB stars show some chemical homogeneity, whatever the
photospheric C/O ratio and stage of evolution of the star.
