The effects of covalent attachment of the electroactive components (i.e. the cation receptor molecules and the tetraphenylborate anions) in durable CHEMFET membranes is described. CHEMFETs for monovalent cations having either one or both electroactive components covalently bound in the membrane matrix exhibit Nernstian responses and good selectivities for Na+, or K+ ions, however, covalent attachment of borate in the membrane results in non-functioning sensors. Durability studies with CHEMFETs modified with polysiloxane membranes which are selective for Na+, K+ and Pb2+ ions show long lifetimes upon continuous exposure to a water stream. Na+ selective CHEMFETs with covalently attached Na+ ionophores and free borate in the membrane are still Na+ selective after 82 weeks of continuous exposure to water. Similar K+ and Pb2+ selective polysiloxane CHEMFETs showed good selectivities for at least 26 and 19 weeks, respectively. Impedance measurements showed that in all cases the lowest membrane resistance and the most stable CHEMFETs were obtained with polysiloxane membranes containing covalently attached ion-selective calixarene-based receptor molecules and free anionic sites.