The development of reliable hydrogen sensing materials for subzero environments is crucial for aviation, cryogenic storage, and hydrogen infrastructure applications. In this study, we investigate tetragonal β-tantalum (β-Ta) thin films at −60 °C to assess their potential for optical hydrogen sensing. In situ X-ray diffraction (XRD) measurements reveal a reversible lattice expansion upon hydrogen exposure, with β-Ta exhibiting a smaller volumetric expansion compared to α-Ta, indicating lower hydrogen solubility. Optical transmission measurements demonstrate a monotonic and fully reversible optical response across a range of hydrogen pressures, free of any hysteresis. However, β-Ta exhibits prolonged response times at low temperatures due to diffusion-limited kinetics, as confirmed by power-law response rate analysis and direct diffusion front measurements. Although β-Ta offers a temperature-independent resolution and structural robustness, its slower response time suggests the need for further microstructural optimizations to enhance hydrogen diffusion. ... Read more