Pseudocapacitance (C-p) of molybdenum with thin passive surface oxides was assessed over a wide potential (E) window (-0.05 to -1.0V vs. Ag/AgCl) at various pH levels in 0.6 M Na2SO4 by means of impedance spectroscopy. The E and pH - dependent variation of C-p was correlated with the surface oxides' stability and H+/Na+ surface/near-surface reversible redox processes. C-p showed a rapid increase in acidic solutions from -0.05 to similar to -0.55V owing to the H+ electrosorption/intercalation which was followed by a sharp decrease in the hydrogen evolution region. A rise in C-p at E < similar to -0.80V was suggested to be associated with either a strong involvement of water at the interface and/or a continuous ingression of Na+ to the porous surface oxides. In neutral and basic electrolytes, the E - dependent C-p remained to be high without a marked maximum. An associated high oxide film resistance indicated that the thermodynamic instability of the surface oxides in neutral/basic electrolytes resulted in an outer dissolute-product's layer, and that the Na+ electrosorption/intercalation occurred primarily with an inner layer of stable surface oxides. Also presented was a minimum potential in the C-p vs. E plot, whose variation was correlated with the open circuit potential. (C) 2014 Elsevier Ltd. All rights reserved.