Chemistry in oxygen-rich circumstellar envelopes.

We have considered the chemistry occuring in the circumstellar envelope surrounding an oxygen-rich AGB star and have specifically modelled 4 sources; R Dor, TX Cam, OH231.8+4.2 and IK Tau. Methane has been assumed to be a parent molecule and the resulting carbon chemistry is investigated. We find that carbon chain molecules up to C_2_H_4_ can be abundant as can CH_3_CN and CH_3_OH. Our model extends previous work by including the chemistry of silicon, chlorine and phosphorus. The presence of CH_4_ as a parent and hence its daughter species CH_3_ and CH_3_^+^ leads to other carbon-bearing species such as H_2_CS, SiCH_2_, H_2_CN and CCl. Sulphur is an important source of electrons in some regions of the envelope. Its main effect is on the abundance of CH_3_^+^ and molecules formed from this radical e.g. CH_3_OH. Where possible we have compared our results to observations and in general there is good agreement. A notable exception is the case of OCS in OH231.8+4.2 whose calculated abundance is roughly 10^4^ times lower than observed.