Early transition metal nitrides are promising materials for use in electrochemical capacitors and as electrocatalysts due to their high electronic conductivities, high surface areas and electrochemical stabilities. In addition to double-layer (electrostatic) charge storage, these materials store charge via a pseudocapacitive mechanism involving charge-transfer reactions. This paper describes electrochemical and in-situ X-ray absorption spectroscopic results that provide important insights regarding the pseudocapacitive charge storage mechanism for molybdenum nitrides in aqueous acids. From analysis of the rest potentials at varying pH values, we concluded that electrons and protons were coupled during charge transfer with a ratio of similar to 2 electrons per proton. Cycling the MO2N electrodes between -0.73 and -0.19 V, the stability window for this material, resulted in the removal of approximately one electron per Mo from a band that was primarily Mo in character. The extended X-ray absorption fine structure spectra indicated minimal changes in the immediate local structure around Mo as the potential was varied. The results are consistent with reversible hydrogen insertion into and extraction from interstitial sites in the Mo2N lattice. Charge storage capacitances in excess of 1500 Fg(-1) are predicted for materials with full occupation of these interstitial sites. (C) 2015 Elsevier B.V. All rights reserved.