Thermally grown oxides on monocrystalline Si wafers have been annealed in a dry nitrogen ambient at temperatures in the range 500–1000°C. MOS structures were made by electron gun aluminium evaporation. The triangular voltage sweep (TVS) method was used to determine the mobile ion concentration ( N m ). A maximum of the N m value was found at anneal temperatures in the range 600–800°C. Etching of the furnace tube in a HF solution reduced both N m and the influence of the anneal temperature on N m . Variation of the duration (30 and 120 min) and the ambient (O 2; O 2 + 2% C 2H 3Cl 3) of the anneal step did not change N m significantly. Experiments with dual anneal treatments showed that the final anneal before the Al metallization was decisive for the level of mobile ion contamination. A qualitative equilibrium model is proposed to explain our results.