Abstract: An on-board gas composition sensor is disclosed for monitoring oxygen content levels in the exhaust gas of an internal combustion engine. The gas composition sensor includes a light source for generating excitation light, e.g. in the 350-525 nm wavelength range. A sensor body of porous high-temperature fluorescent inorganic oxide ceramic exposed to the exhaust gas emits an optical fluorescence signal responsive to oxygen content in the exhaust gas upon exposure to the excitation light at 400.degree.-650.degree. C. A florescence detector receives the optical fluorescence signal from the sensor means and generates an exhaust gas oxygen content output signal in response. Fiber-optic cable can be employed for transmitting excitation light to the sensor body from a remotely located light emitter. The same or a separate fiber-optic cable can transmit the optical fluorescent signal from the sensor body to the fluorescence detector also at a remote location.