A hydrogen sensor has been developed based on a modification to the Wacker chemistry used for reversible detection of carbon monoxide. Critical elements of the modified chemistry are metallic palladium stabilized by a heteropoly acid, supported on a porous Vycor™ matrix. In inert environments such as helium or nitrogen the sensor responds over a range of hydrogen concentrations from ppm levels to several percent by volume. The reversible response is measured as a darkening of the sensor within the visible spectrum. This mode of operation lends itself to fiber optic coupling of the sensor to light source and detector. Response times are typically only a few seconds except at very low concentrations, where mass-transfer effects become important. The sensor is fully reversible and recovers rapidly. The behavior of the sensor has been modeled as an equilibrium process between gas-phase hydrogen and two phases of hydrogen-palladium solution. This model permits linearization of the sensor signal over at least three orders of magnitude in hydrogen concentration. In the presence of atmospheric oxygen the sensor exhibits complex behavior for hydrogen concentrations below about 1,000 ppm, suggesting catalytic oxidation of the hydrogen. At higher hydrogen concentrations the response is comparable to that observed in an inert gas environment.