Based on the one-chamber fuel cell design by Iwahara a catalytic methane sensor has been developed. The working principle of this sensor is based on the difference in catalytic properties of two electrodes for the CO2 reforming reaction of methane. The sensor is based on a high-temperature proton conducting electrolyte, i.e. SrCe0.95Yb0.05O3-alpha or CaZr0.9In0.1O3-alpha. At 500 degreesC a linear sensor response on the methane partial pressure has been found for a Ru/SrCe0.95Yb0.05O3-alpha/Pt cell. This cell, however, shows poor long-term stability. The long-term stability of the Ru/SrCe0.95Yb0.05O3-alpha/Pt cell is improved using a more stable electrolyte material, i.e. CaZr0.9In0.1O3-(alpha)(CZI10). Further improvement of the long-term stability of the sensor is achieved using a nickel-CaZr0.9In0.1O3-alpha CERMET (Ni-CZI10) electrode. The sensor response of a Ni-CZI10/CaZr0.9In0.1O3-alpha/Pt cell is found to be linear at 600 degreesC and 700 degreesC, respectively. The temperature dependence of both the Ru/SrCe0.95Yb0.05O3-alpha/Pt and the Ni-CZI10/CaZr0.9In0.1O3-alpha/Pt cell can be explained by the temperature dependence of the catalytic activity of the electrode materials used. This confirms that the obtained EMF is established by a catalytic activity difference between both electrodes. The power output of a Ni-CZI10/CaZr0.9In0.1O3-alpha/Pt cell is also determined. A combined sensor-fuel cell would have the advantage that it is able to detect the fuel concentration in the gas and, therefore, correct in-situ for fluctuations in the fuel concentration. The power output of the Ni-CZI10/CaZr0.9In0.1O3-alpha/Pt cell, however, is found to be 0.01 mW . cm(-2). This low power output, with respect to values reported in literature for the one-chamber fuel cell, can be explained by the relatively thick electrolyte used, the electrode materials chosen, and the use of the reforming reaction of methane instead of the partial oxidation of methane. However, the feasibility of the combined sensor-fuel cell has been demonstrated.