The electrochemical behaviour of pure molybdenum in 85% H3PO4 at high positive overpotentials was studied by d.c, polarization, a.c. impedance and photoelectrochemical techniques. It was found that passivation of the electrode occurred via formation of a barrier-like semiconductor film. Photocurrent spectra point to the identification of this film as MoO3. A.c. impedance measurements in the passive region were consistent with high-field migration controlled film growth and relaxation of the charge carrier density. The surface charge approach proposed earlier was used to compute the kinetic and structural characteristics of the barrier layer : steady state electric field strength, capture cross-section for positive carriers, half-jump distance for a hopping ion, transference number of negative defects and current efficiency for film formation.