Abstract: A sensor for measuring the activity of ions by means of ion-selective membranes (4) improves the measuring possibilities and reduces the dimensions of the sensor in that a plurality of such membranes are held in a common carrier (3) and are electrically insulated and protected against the penetration of moisture. Shunts (2) are provided in a sensor body (1) of the carrier. The carrier and sensor body are connected to one another. The invention is also directed to a method for producing such a sensor. Moreover, the invention is concerned with sensors and a respective arrangement for hemodialysis.