Paramagnetic defects in atomic layer deposition grown aluminium oxide thin films have been studied using electron paramagnetic resonance. Initial spectra indicate the presence of Si-db, Pb0 and Pb1 defects, previously observed in Si/SiO2 structures. We show that the Si-db defect is located in the substrate only. We quantify the unpassivated P b-type defect density and show that this can be reduced by a pre-deposition nitridation step. However, forming gas annealing at temperatures up to 550 °C causes no further reduction in defect density; this may be related to the low deposition temperature, which causes a spread in passivation activation energies.