The anodic oxidation of molybdenum in H2SO4 solutions (0.5-5.0 M) was studied, emphasizing transpassive dissolution. Voltammetric experiments indicated that transpassive dissolution was kinetically controlled, and that a secondary passivation process took place under transport control. Steady state polarization curves in the transpassive dissolution region exhibited a two-slope Tafel-like behaviour, suggesting the concurrence of two parallel reaction paths. Ac impedance measurements revealed a total of four time constants, implying the existence of three stable reaction intermediates. A quantitative model of the interface, comprising five reaction steps involved in two parallel pathways, is proposed. This model is able to reproduce both the stationary polarization curve and the general features of the impedance spectra.