A model is proposed on the origin of the reversible redox transitions of molybdenum (Mo) and its oxides. The major and minor redox transitions are proposed to be associated with two thermodynamically non-equivalent adsorption sites at the structured surface oxides. Anion interaction/adsorption was found to be a decisive factor.. The criterion is based on the combination of evidences and assumptions and is derived from the variations observed in the two redox transitions during a series of cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) transient studies. The experiments were conducted using different electrolytes with dissimilar anions (Cl-, SO42- and ClO4-) and spanned a range of different pH values. The results described herein constitute an important step towards understanding the interfacial electrochemical processes of Mo and its oxides, and will be of use for a wide range of their applications. (C) 2014 The Electrochemical Society. All rights reserved.