Ion sensor based on differential measurement comprising an ISFTET-REFET pair wherein the REFET is defined by a structure composed of an ISFET covered by a microreservoir where an internal reference solution is contained. The sensor comprises a first and a second ion-selective field effect transistor, an electrode, a substrate on the surface whereof are integrated the two transistors, connection tracks and the electrode and a structure adhered on the first ion-selective field effect transistor which creates a microreservoir on the gate of said first transistor, with the microreservoir having a microchannel which connects the microreservoir with the exterior and the microreservoir being filled with the reference solution.