The potential of discrete and distributed fiber-based sensors exploiting the Rayleigh scattering signature of doped amorphous silica is investigated for the real time monitoring of molecular hydrogen (H2) detection. We showed that the impact of the refractive index changes induced by the H2 diffusion into the silica host matrix can be used to detect and quantify this gas presence through two approaches: first via the related fiber length variation and second through the observed spectral shift. Comparing the obtained results with H2 diffusion calculations, we can estimate the sensor sensitivity thresholds to be ∼1016 nmolecule cm−3 for the distributed measurements (spatial resolution better than 1 mm) and below ∼1019 nmolecule cm−3 for the discrete-one. The presented architecture of the sensor is well adapted to the monitoring of slowly evolving H2 concentrations such as the ones expected in nuclear waste repositories as the time response of the sensor remains limited by the diffusion of the gas within the optical fiber. These threshold values and time responses can be easily improved by optimizing the length, the composition and/or the geometry of the sensing fiber.