A new construction method of a PVC-matrix membrane ISFET-based urea sensor is described and compared with a pH ISFET-based urea sensor which uses a classical immobilization procedure on a nylon net. The response slope obtained with data from the pH ISFET-urea sensor is 57 mV/decade over a linear working range of 10-4-3 × 10-3 M urea with a lifetime over 20 days. This sensor, applied in a flow-injection system, presents a sensitivity of 60 mV/decade and shows an acceptable sampling rate of 35 samples/h. Although its lifetime is shorter under dynamic conditions the membrane is easily changeable in the flow system. The new sensor reported is based on an ammonium ISFET with the PVC-matrix membrane cast on a graphite/epoxy composite. The enzyme is deposited and covalently fixed onto the PVC membrane by means of a fine spraying of glutaraldehyde. This sensor shows a sensitivity of 42 mV/decade over a linear working range between 4 × 10-5 and 10-2 M urea and a long-term stability of one week. © 1993.