Local structural arrangements and stabilities of oxygen and hydrogen-related defects in proton-conducting LaP3O9 were studied using first principles calculations. When an oxygen was removed from LaP3O9, the crystal lattice was significantly distorted. Resulting structural arrangements considerably depended on the oxygen deficient site, and phosphate ions tended to condense by sharing a corner oxygen of PO4 tetrahedra. On the other hand, when a proton was introduced, the proton was located at the interstitial sites positioned approximately 1 angstrom away from the nearest oxygen forming an O-H bond. The LaP3O9 lattice was only slightly distorted even after introduction of an interstitial proton. Based on the calculation results, the stabilities of the defects under moisturized conditions was discussed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.