The design of durable sodium-selective CHEMFETs based on the covalent attachment of a sodium-selective ionophore and tetraphenylborate anions to a polysiloxane membrane matrix is described. Simulations of the membrane potential of such CHEMFETs using an extended version of the model developed previously in our group, revealed that a membrane with a reduced mobile ionophore and completely immobilized anionic sites should give a sub-nernstian response owing to a counteracting diffusion potential. CHEMFETs with all possible combinations of free and covalently bound ionophore and borate anions were prepared and the effect of covalent binding on the sensing behaviour was studied. The attachment of both electroactive components to a polysiloxane membrane matrix results in CHEMFETs that respond to Na+ activities in aqueous solution with good selectivity, and an almost nernstian slope (56.7 mV decade¿1). The polarity of the membrane plays a crucial role. The durability is improved by the covalent attachment of the electroactive components (more than 90 days).