Abstract: The invention relates to a lithium ion-selective membrane, including a polymer carrier, a plasticizer, a conductive compound and a lithium ion specific ionophoric compound. The invention consists in that said ionophoric compound is dibenzyl-14-crown-4 and derivatives thereof, and represents between 0.5 and 3% by weight of the total composition of the membrane, the polymer carrier representing 25 to 30% by weight of the total composition of the membrane, the plasticizer from 65 to 72% by weight of the total composition of the membrane and the conductive compound from 0.2 to 1.5% by weight of the total composition of the membrane. Said invention can be used to determine the lithium ion concentration of a fluid, such as a primary fluid for a pressurized water reactor in a nuclear power plant.