The effect of high pressure (1.2. GPa)-high temperature (up to 1400 K) treatment (HP-HT) on the structural properties of GaMnAs/GaAs, AlGaAs/GaAs and Si/SiO2/Si (SOI) structures was studied by high resolution X-ray diffractometry. GaMnAs layers remain strained after the HP-HT treatment, while it results in anisotropic strain relaxation of AlGaAs. The change in lattice parameters of A(III)B(V) layers is explained as an effect of relaxation of the misfit strain via creation of misfit dislocations and of other extended defects, as well as by diffusion of Mn or Al to dislocations. The treatment of SOI structures resulted in increased las compared to the effect of annealing at 10(5) Pa) strain between the Si top layer and the Si bulk.