A process for determining a content of hydrogen in a fluid medium includes contacting the fluid medium with a sensor. The sensor has a housing enclosing a chamber containing an ionic liquid electrolyte, a window which is permeable to hydrogen and positioned in an opening in the housing, and electrodes in contact with the ionic liquid electrolyte in the chamber. Hydrogen is allowed to pass through the window from the fluid medium into the electrolyte and the sensor is heated. Temperature and pressure of the fluid medium is determined and electrical potential is applied to the electrodes. The method also includes measuring current flow. The sensor can be used to observe hydrogen concentration by voltammetry. The method and sensor may be used for measuring downhole hydrogen content, monitoring fiber-optic cables for damage by hydrogen, corrosion monitoring, and in small-scale process plants where hydrogen is part of a gas stream.