An active core fiber optic sensor has been developed for detecting trace H 2 S in high-temperature gas samples. This sensor uses a cadmium oxide doped porous silica optical fiber prepared with a sol–gel process as a transducer. When this porous silica optical fiber was exposed to a gas sample at high temperatures, trace H 2 S in the sample diffuse into the porous fiber, and react with the doped cadmium oxide to form cadmium sulfide. The optical properties of this formed compound inside the porous silica optical fiber was investigated with fiber optic spectrometric methods. It was found that cadmium sulfide formed inside the porous silica optical fiber absorbs UV light with peak absorption wavelength at around 370nm. This absorption signal was observable at a high temperature (450°C). The formed CdS inside the fiber after H 2 S exposure emits strong fluorescence at room temperature with peak emission wavelength at around 500nm. However, this fluorescence signal was quenched at the high temperature (450°C), and the fluorescence emission property can only be used for detecting trace H 2 S at low temperature region. The UV absorption based sensor has been calibrated for detecting trace H 2 S in a nitrogen gas sample, and a linear calibration curve was obtained. This sensor can be used for detecting trace H 2 S in a high temperature gas sample down to sub-ppm level. Matrix gas components of gasification-derived fuel gas, such as H 2 , CH 4 and CO, were found not interfering with the sensor for detecting H 2 S. This sensor is expected having the potential of application for monitoring the process of cleaning fossil fuel derived fuel gas.