The paper reviews the problems related to BOX high-temperature instability in SOI structures and MOSFETs. The methods of bias-temperature research applied to SOI structures and SOI MOSFETs are analysed and the results of combined electrical studies of ZMR, and SIMOX SOI struc- tures are presented. The studies are focused mainly on elec- trical discharging processes in the BOX at high temperature and its link with new instability phenomena such as high- temperature kink effects in SOI MOSFETs.