We propose to search the circumstellar envelopes of a number of
evolved oxygen-rich stars for the presence of carbon-bearing
molecules.  Our primary molecule is CH4, but we shall also
search for CO2 and C2H2. The chosen objects all show HCN emission
at radio wavelengths, and some have CN, HNC, CS and H2CO present
as well. These detections have been somewhat of a surprise since
all carbon is predicted to be tied up in the tightly-bound CO
molecule.  Detailed chemical kinetic models of the photochemical
regions of the envelopes have been developed and suggest that some
other source of reactive carbon must be present.  Key molecules in
this regard are CH4, C2H2 and CO2.  The first two may be formed by
the interaction of gas and dust deep in the envelope, while CO2 can
be formed in warm (300K) gas in the outflow.  We shall also obtain
spectra of the two S-type stars, W Aql and Chi Cyg, showing HCN
emission, to study the influence of the O/C ratio on the chemical
composition.  The observational data will be interpreted with the
aid of the latest chemical kinetic models of O-rich circumstellar
envelopes.