A new method to functionalize silanol-terminated polysiloxanes is presented. The method can be used for the realization of tailor-made ion-selective membranes. A siloxane copolymer containing polar cyano groups combined with a condensation-type cross linking overcomes the earlier encountered problem of high membrane resistance. After modification of this material with methacrylate groups, a photocurable polysiloxane was obtained. Potassium- and calcium-selective ISFETs with excellent sensitivities and selectivities (log KK,i<-3.5, i=Na+, Mg2+, Ca2+-log KCa,i<-3.2, i=Na+, K+, Mg2+) could be made with this material using photolithography. Also quaternary ammonium ion-exchange groups could be coupled to the polymer using the same technique. Using this modified polysiloxane we have been able to realize nitrate-sensitive ISFETs with a slope of 57-58 mV/decade, and a selectivity versus Cl- of log KNO3, Cl=-2.