A sensor device consisting of a MOSFET with an iridium oxide gate contact and denoted an (MOSFET)-M-E is presented. When the gate of this device is in contact with an electrolyte, the iridium oxide can take part in a redox reaction, enabling thermodynamic equilibrium between the electrons in the iridium oxide and protons in the solution. The chemical potential of the electrons in the bulk of the iridium oxide can be changed by oxidation or reduction of the material and is related closely to the iridium oxide work function. Since the threshold voltage, V-T, of the MOSFET depends on the work function difference between the gate contact and the silicon bulk, it is influenced also by the redox reaction. So if this sensor is connected to the appropriate amplifier circuit, the redox reaction induces changes in the output signal of the system due to changes in the threshold voltage, VT, of the MOSFET. It is shown on a theoretical basis how the output signal is influenced by the redox reaction and this description of the device is supported by measurement results.