We report a systematic and comprehensive computational study of reconstructed GaN and InN surfaces in various orientations, including the polar c plane as well as the nonpolar a and m planes. Two distinct microscopic origins for Fermi-level pinning are identified, depending on surface stoichiometry and surface polarity. We also explain the source of the electron accumulation that has been universally observed at InN surfaces. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions. (c) 2006 Elsevier B.V. All rights reserved.