The anodic oxidation of molybdenum in acidic sulphate solutions (pH from 0 to 3) is investigated, emphasizing transpassive dissolution. Steady state polarization curves exhibit a two-slope Tafel behaviour suggesting the concurrence of two parallel reaction paths. Ac impedance measurements reveal a total of five time constants, implying the existence of four reaction intermediates. The occurrence of the fourth intermediate was found to be characteristic for pH greater than or equal to 1, whereas the first three are found throughout the pH range studied. A quantitative model of the metal/solution interface is advanced comprising six reaction steps involved in two parallel pathways. The values of the rate constants of the reaction steps are found to be dependent on pH.