Microstructure and related properties of hydrogenated silicon samples, Si:H, treated at high-temperature (HT) up to 1270K under hydrostatic argon pressure (HP) up to 1.1GPa are investigated. To prepare Si:H, Czochralski grown 001 oriented single crystalline Si wafer with 50nm thick surface SiO2 layer was heavily implanted with hydrogen using the immersion plasma source of hydrogen ions with ... [Show full abstract] energy 24keV.The surface of HT–HP treated Si:H was characterised by scanning electron microscopy. Reflectivity pattern measurements in the wavelength range of 350–2000nm have been performed to analyse their surface and bulk properties. The volume averaging method for a model of layer-like structure has been used to simulate the HT–HP treated Si:H. The analysis of Si:H samples suggests the multi-layer structure composed of Si, Si:H, SiO, SiO2, and of porous Si layers in the sub-surface region. The porous Si:H samples model is in good consistency with experimental data from reflectance measurements.