High temperature annealing of SOI wafers in non-oxidized ambient leads to internal Buried Oxide (BOX) dissolution. The underlying mechanisms and kinetics of this effect are discussed. High quality SOI wafers with very thin BOX down to 2nm are demonstrated utilizing optimized annealing conditions. Hybrid SOI/bulk wafers are obtained by the new process applying silicon nitride mask on the wafer surface. Stability of SOI and Si3N4/SOI systems at high temperatures is discussed and optimized process window is determined.