The construction of chemical sensors requires the integration of a sensing molecule and a transducing element. We have developed an integrated CHEMFET on the basis of an SiO2 ISFET by covalently linking a hydrophilic polyHEMA layer to the SiO2 gate oxide and a hydrophobic sensing membrane that contains the sensing molecule to this polyHEMA layer. The optimized material for the sensing membrane is a terpolymer of different siloxanes. The sensing molecules that we have studied were designed for a high K+, Na+, Ca2+ and Ag+ selectivity. Typical examples of such molecules are hemispherands or calixarene crown ethers for K+ and tetra-O-alkylated calixarenes in the cone conformation for Na+ or Ag+. The sensing molecules can be present as mobile hydrophobic species or they can be covalently attached to the sensing membrane. We have compared both possibilities and in both cases a durable sensor can be obtained. The sensors that have covalently attached receptors exhibit fast response times and are chemically stable for prolonged periods (>200 days).