A sensor for hydrogen in a fluid medium has a chamber for electrolyte with a window which is selectively permeable to hydrogen to allow hydrogen to pass from the fluid medium under test into the electrolyte in the chamber. A plurality of electrodes in contact with the ionic liquid electrolyte are used to observe hydrogen concentration by voltammetry. The electrolyte is an ionic liquid. Applications where such a sensor may be used include a wellbore tool for measuring the content of hydrogen in a subterranean fluid, monitoring of fiber-optic cables for damage by hydrogen, corrosion monitoring, and small-scale process plant where hydrogen is part of a gas stream.