Electron paramagnetic resonance was conducted on aluminium oxide films deposited by atomic layer deposition on (100)Si. Multiplet spectra are observed, which can be consistently decomposed assuming the presence of only P b0 and Pb1 centres, which are well known in Si/SiO 2 structures. Al2O3 films deposited on HF-treated (100)Si exhibit unpassivated Pb0 and Pb1 centres, with concentrations of (7.7 ± 1.0) × 1011 cm-2 and (8 ±3) × 1010 cm-2, respectively. Rapid thermal annealing of the substrate in NH3 prior to film deposition reduces the unpassivated Pb0 concentration to (4.5 ± 0.7) × 1011 cm-2. Forming gas annealing at temperatures in the range 400-550°C causes no further reduction in defect density; this may be related to a spread in passivation activation energy, associated with low-temperature deposition.