Impurity-related electronic states in high-purity high-resistivity n-type floatzone (FZ) Si have been studied by deep level transient spectroscopy (DLTS). FZ-Si with a doping concentration of ∼1 × 10 12 cm −3 was chemically etched in 10% diluted hydrofluoric acid (HF) prior to thermal deposition of a Schottky gold contact. DLTS measurements of the as-prepared diode reveal the presence of a deep electronic trap at 0.52 eV below the conduction band. Depth profiling measurements show that the trap concentration increases towards the surface, indicating that it is due to an impurity diffusing from the surface. Heat treatments for 15 min at 150-300 • C lead to annealing of the 0.52 eV trap and formation of a 0.17 eV trap that is uniformly distributed versus depth. A reverse formation of the 0.52 eV trap with a uniform depth distribution, accompanied by annealing of the 0.17 eV trap, is observed after storage of the sample at room temperature for 20-50 days. A subsequent heat treatment at 300 • C results in the repeated annealing of the 0.52 eV trap and formation of the 0.17 eV trap, which is followed by the reversed formation of the 0.52 eV trap and annealing of the 0.17 eV trap at room temperature (RT). Several such annealing/formation cycles can be performed with these centres with a good reproducibility of the data. It might be suggested that both centres are related to hydrogen, although the exact origin is not identified.