The influence of CO(2) and acetic acid on the response of ISFET sensors with PVC and photocured polyurethane polymer membranes with valinomycin as an ionophore was assessed. Experimental results show that the presence of these compounds has no effect on sensors parameters even after prolonged soaking in a water solution. Using a photocured polyurethane polymer as an ion-selective membrane matrix for an ISFET, a carbonate ion sensor was developed with hexyl-p-trifluoroacetylbenzoate (HE) as an ionophore. Effect of cationic and anionic lipophilic additives on the sensors response was studied. Sensors with the optimized membrane composition based on HE (7.9%, w/w) and tridodecylmethylammonium chloride (5.7%, w/w) show sensitivity of 27-30mV per decade of carbonate ion concentration, sufficient selectivity in front of chloride ions, and a lifetime of 3-5 months.