Anisotropic nanostructured porous Pd films are fabricated using oblique angle deposition in vacuum on a glass substrate. They display a dichroic response, due to localised surface plasmon resonances (LSPR) within the nanoparticles forming the film, dependent on the incident light polarisation. Ultrasensitive hydrogen sensing is reached by using these films in conjunction with a differential optical technique derived from the reflectance anisotropy spectroscopy. The evolution of the samples' optical responses is monitored during the formation of Pd hydride in both the dilute α-phase and the dense β-phase, whilst the samples are exposed to different concentration of H 2 in Ar (from 100% H 2 to a few ppm). The measurements are performed at a single wavelength in the visible range and at 22°C. The results show that a quantitative measurement of the hydrogen concentration in a carrier gas can be measured throughout the concentration range. The limit of detection is 10 ppm and the time for detecting the presence of H 2 in the carrying gas is below one second at concentration down to 0.25% of H 2 in Ar. Furthermore, the optical anisotropy of the samples and its evolution with exposure to H 2 are correctly reproduced with an effective medium theory.