Phosphorus oxide (POx) capped by aluminum oxide (Al2O3) has recently been discovered to provide excellent surface passivation of crystalline silicon (c-Si). In this work, insights into the passivation mechanism of POx/Al2O3 stacks are gained through a systematic study of the influence of deposition temperature (Tdep = 100–300 °C) and annealing temperature (Tann = 200–500 °C) on the material and interface properties. It is found that employing lower deposition temperatures enables an improved passivation quality after annealing. Bulk composition, density, and optical properties vary only slightly with deposition temperature, but bonding configurations are found to be sensitive to temperature and correlated with the interface defect density (Dit), which is reduced at lower deposition temperature. The fixed charge density (Qf) is in the range of + (3–9) × 1012 cm–2 and is not significantly altered by annealing, which indicates that the positively charged entities are generated during deposition. In contrast, Dit decreases by 3 orders of magnitude (∼1013 to ∼1010 eV–1 cm–2) upon annealing. This excellent chemical passivation is found to be related to surface passivation provided by hydrogen, and mixing of aluminum into the POx layer, leading to the formation of AlPO4 upon annealing.