The mathematical tools necessary to describe quantitatively the chemical processes that dictate the performance of exhaust oxygen sensors are developed. Such sensors are used commonly to monitor exhaust streams generated by internal-combustion processes. Calculated results compare well with available experimental results, although several open questions are identified that require more experimental data. The mathematical formalism for describing the transport of gaseous species through the porous spinel structure protecting the platinum electrode on the exhaust side of the sensor is developed based on the Stefan-Maxwell equations. The kinetic processes occurring at the interface formed by the platinum electrode and the spinel structure, including the oxidation of hydrogen and carbon monoxide and various adsorption-desorption reactions, enter as boundary conditions for the transport equations. The analysis enables one to calculate the sensor’s voltage response as a function of the air-to-fuel ratio lambda and to investigate phenomena such as the magnitude of the voltage jump in going from rich to lean gas mixtures and the lambda value at which this jump occurs.