The substoichiometric {111}, {110} and {100} surfaces of UO2 and PuO2 are studied computationally using two distinct yet related approaches based on density functional theory; the periodic electrostatic embedded cluster method (PEECM) and Hubbard-corrected periodic boundary condition DFT. First and second layer oxygen vacancy formation energies and geometries are presented and discussed; the energies are found to be substantially larger for UO2 vs PuO2, a result traced to the substantially more positive An(IV)/An(III) reduction potential for Pu, and hence relative ease of Pu(III) formation. For {110} and {100}, the significantly more stable dissociative water adsorption seen previously for stoichiometric surfaces [J. Nucl. Mater. 2016, 482, 124–134; J. Phys. Chem. C 2017, 121, 1675-1682] is also found for the defect surfaces. By contrast, vacancy creation substantially changes the most stable mode of water adsorption on the {111} surface, such that the almost degenerate molecular and dissociative adsorptions on the pristine surface are replaced by a strong preference for dissociative adsorption on the substoichiometric surface. The implications of this result for the formation of H2 are discussed. The generally very good agreement between the data from the embedded cluster and periodic DFT approaches provides additional confidence in the reliability of the results and conclusions.