Abstract: Hydrogen gas sensors with low cost, fast response time, large detection range, high sensitivity, and excellent limit of detection are described. The devices include a composite formed on a nano-scale that both absorbs hydrogen and exhibits magneto-optical effects. Sensor readout is based on magneto-optical effects, which also may be enhanced through plasmonic coupling. The hydrogen sensors are fast and sensitive, as well as resistant to surface poisoning from common contaminants, such as carbon monoxide.