The potentiodynamic oxidation of zirconium in different electrolytes was investigated by means of in-situ ellipsometry and the oxide surfaces were characterized by scanning electron microscopy examination. The different models suitable for the description of the metal oxide electrolyte interface are analyzed. The forming electrolyte affects both the optical constants and the structure of the oxide filMs. FilMs formed in H2SO4 grow as single isotropic layers, while films formed in NaOH show some degree of anisotropy. The oxides formed in H3PO4 have a two-layer structure in which the outer layer has a low refractive index. The DELTA-psi profiles recorded during oxidation in HNO3 reflect different stages of oxide dissolution and metal roughening which can be simulated with appropriate two-double-layer models.