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.