Mobile protons are generated at the buried oxide (BOX)/top-Si interface in Si/SiO2/Si structures by H atoms which permeate the top Si layer during heat-treatment in H-2. About 1/1000 of the arriving protons form hydroxonium-like configurations with O atoms having a sufficient effective negative charge, which are associated with "strained" Si-O bonds, and by transferring an electron from the H atom to the silicon. The strained bonds are generated thermally during the high temperature processing as the relaxation of the confined BOX is hindered and by compressive mechanical stress on the BOX, especially at the edges of the top Si layer. The interaction between protons and Si/SiO2 interfaces is limited or even nil from the standpoint of generating interface states. The probable reason is that the Si/SiO2 interfaces of structures with a confined oxide layer (BOX) are different from those of conventional structures with unconfined oxide.