Functionalized polysiloxanes can be used as sensing membranes in ion-sensitive field effect transistors (ISFETs). A new and reproducible route for obtaining well-defined polysiloxane copolymers-the anionic copolymerization of cyclotetrasiloxanes with different side groups (one of which is shown in the Figure)-is reported and the characterization and use of the copolymers in CHEMFETS described.