NOVELTY - A hydrogen sensor (10) comprises a composition (1) containing a gas permeable matrix material, intermixed with and encapsulating at least one chemochromic pigment that changes color in presence of hydrogen gas. The sensor further comprises a support/overcoat layer (2), and the composition is disposed on the support/overcoat layer. The composition optionally includes an accelerant or contrast additive; and a reversibility enhancing agent encapsulated in the gas permeable matrix material.    USE - For sensing hydrogen gas; useful in hydrogen sensing applications such as e.g. smart paints, tapes, articles and fabrics for making gloves and masks, to warn hydrogen leaks; useful in calorimetric hydrogen sensor system; useful as an integrated sensor fabricated on a chip provided with electronic components; useful in fiber optic sensing heads to provide remote detection; useful in automatic sensing system to provide naked-eye human visual-sensing; useful in hydrogen detection system positioned along a hydrogen supply line that provides fuel to an electrochemical generator such as proton exchange membrane (PEM) fuel cell; useful for detecting hydrogen leaks in industries, during reduction of metal oxides (e.g. iron ore), ammonia synthesis, production of hydrochloric acid, methanol and higher alcohols and aldehydes, and hydrogenation of petroleum, coal, oil shale and edible oils; and useful in space, fuel transportation, oil refineries and ammonia and hydrogen plants.    ADVANTAGE - The matrix material provides high hydrogen permeability, which permits fast permeation of hydrogen gas. The high gas permeability matrix material allows the hydrogen sensor composition to be used in thicker segments, and with lower concentrations of active pigment as compared to prior-art hydrogen sensors, while retaining the rate and extent of pigment color change similar to free pigment, even at extreme temperatures. For example, the hydrogen sensor provides hydrogen detection color change at temperatures as low as -40degreesC. The color change of hydrogen sensor is reversible due to incorporation of reversibility enhancing agents in the composition. In addition, the encapsulating matrix provides enhanced protection, preferably to the chemochromic pigment exhibiting high water solubility, against weather and environmental contaminants e.g. contaminants including moisture. The hydrogen sensor provides naked-eye human visual-sensing. The hydrogen sensor provides a high level of selectivity to hydrogen, as compared to various prior-art hydrogen sensors.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic for a two layer hydrogen sensor composite.    composition layer containing a gas permeable matrix material and chemochromic pigment (1)    overcoat layer (2)    hydrogen sensor. (10)