Electrochemical quartz crystal microbalance studies of MnOx-coated carbon nanofoams reveal that charge-compensation mechanisms associated with MnOx pseudocapacitance in mild aqueous electrolytes are dominated by anion insertion rather than more commonly reported cation ejection. Specific charge-compensation behavior depends on such factors as electrolyte composition, nanofoam pore size, and polarization amplitude. For example, MnOx carbon nanofoams with average pore sizes of 5-20 nm, cycled in 2.5 M LiNO3, reveal a kinetically-hindered, mixed anion-cation charge-compensation mechanism, whereas the same nanofoam cycled in 2.5 M NaNO3 shows only anion association. Nanofoams with larger pores (10-200 nm) that are cycled in 2.5 M LiNO3, reveal anion-only charge compensation. Our results demonstrate that critical new insights on charge-storage mechanisms are achieved using EQCM methods, even when analyzing practical, macroscale electrodes such as carbon nanofoams. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.