Effect of annealing up to 1400 K under argon pressure up to 1.2 GPa (HT-HP treatment) on silicon implanted with hydrogen (Si:H), oxygen (Si:O) and on silicon-on-insulator (SOI) structure was investigated by X-ray methods and transmission electron microscopy (TEM). The results obtained for the HT-HP treated samples were compared with that for the samples annealed at atmospheric pressure (10 5 Pa). For Si:H and Si:O samples the HT-HP treatment influences strain state of the Si top layer much stronger than annealing at atmospheric pressure. The pressure treatment prohibits the hydrogen diffusion towards the surface region. The as-bonded SOI structures indicated presence of hydrogen and defects. The mosaic-like structure with high defect concentration was observed also for SOI treated at 1370 K–1.2 GPa, while annealing at 10 5 Pa resulted in improved perfection of the Si top layer.