  The gas-phase non-equilibrium chemistry of the inner wind of the oxygen-rich
Mira variable IK Tau (NML Tau) is investigated using a physio-chemical model
describing the periodically shocked gas close to the stellar photosphere. We
predict the formation in large amounts of a few carbon species in the inner
envelope, in particular CO$_2$, which has been recently detected with ISO in
the spectra of several oxygen-rich semi-regular and Mira stars. The theoretical
abundances are also in excellent agreement with values derived from millimeter
and sub-millimeter observations, pointing to the fact that some carbon species
in oxygen stars do form from shock chemistry in the inner layers and travel the
envelope as ``parent'' species.
