Abstract: The invention relates to a hydrogen sensor with a radiation source, by means of which electromagnetic radiation is radiated onto a sensor medium, wherein the sensor medium has a transmission coefficient that varies as a function of the concentration of hydrogen in the environment of the sensor medium, and with a detector that detects at least a portion of the radiation transported through the sensor medium. The hydrogen sensor according to the invention is characterized by the fact that a reflector is provided that reflects the radiation transmitted through the sensor medium back to the sensor medium. The hydrogen sensor according to the invention is further characterized by the fact that the sensor medium incorporates clusters containing or consisting of a palladium alloy, yttrium, scandium, at least one lanthanide, at least one actinoide, tungsten oxide and/or vanadium oxide, and/or a mixture or compound of these materials.