The authors report on the micostructural changes in HT (high-temperature-implanted) SIMOX (separation by implanted oxygen) at various stages in the ramping process by simulating the thermal treatment with two-hour anneals at intermediate temperatures. The wafers studied were implanted to 1.8 × 1018 cm-2 at 200 keV at a temperature of 620°C. To simulate the effect of the thermal ramping cycle, wafers were annealed for 2 hours at 50°C intervals from 800°C to 1100°C. Cross section samples were studies with conventional transmission electron microscopy (TEM) techniques at 200 keV. Major microstructural changes are shown to occur in SIMOX during the thermal ramping cycle between temperatures of 900°C and 1100°C. These changes strongly affect, possibly even control, the final defect density and buried oxide microstructure, even prior to the final high temperature anneal. Dislocation formation in SIMOX occurs during thermal ramping, probably between 1000°C and 1100°C. This suggests that it may be possible to further reduce dislocation densities of 106 cm-2, as found in HT SIMOX, by altering the thermal ramping cycle.