SIMOX wafers annealed at high temperature in argon or nitrogen ambient are compared in terms of film, oxide, interface, and device properties. This evaluation is conducted by correlating pseudo-MOSFET data in unprocessed wafers with the performance of n- and p-channel transistors. Characterization with the Ψ-MOSFET technique reveals a pile-up of nitrogen near the buried Si -SiO2 interface, where the electron mobility is degraded. MOSFET characteristics are dramatically affected by annealing in nitrogen with large shifts of front and back channel threshold voltages, enhanced leakage currents, and channel doping modification. Nitrogen-induced donors are suspected to be responsible for doping compensation in n-channels (leading to full-depletion mode operation) and overdoping in p-channels. By contrast to argon ambient, which has no major drawbacks, nitrogen ambient can be unsuitable for annealing of SIMOX wafers.