A potentiometric hydrogen sensor was fabricated by covering one of the planar platinum electrodes on a solid yttria stabilized zirconia electrolyte with an oxide catalyst mixture containing CuO/ZnO/Al2O3 in the molar ratio of 7:10:3. A mathematical model was developed to describe the sensing mechanism and operating characteristics of the sensor, incorporating the faradaic reactions and other rate processes including diffusion, adsorption, and catalytic reaction in the oxide layer. Good response characteristics and sensitivity were obtained over the temperature range of 688 to 773 K for air-hydrogen mixture containing 0 to 0.145% of hydrogen by volume. The sensor response over the concentration range of 0 to 0.0145% of hydrogen in air was satisfactorily described by the model assuming ideal Langmuir adsorption characteristics of homogeneous active sites. However, for the complete concentration range of 0 to 0.145% of hydrogen, the experimental data were better described by the model when nonideal adsorption and heterogeneous active sites are assumed.