The open circuit potential, capacitance and resistance techniques were used to investigate the behaviour of two types of surface zirconium oxides in pure 0.5 N H2SO4 and in the presence of additives of K2CrO4, KI, FeSO4 and HCOOH. The air-formed preimmersion oxide film grows in all solutions following a logarithmic rate law. Depending on the nature of the additive the oxide grows directly by oxidation of the surface by the oxidizing additive (K2CrO4) or indirectly by mediating the oxidation by atmospheric oxygen in the presence of reducing additives. Anodically formed zirconium oxide films dissolve in all solutions. The results of impedance measurements indicate that adsorption of HCOOH activates the oxide surface owing, possibly : to its hydrophilic nature while iodide adsorption diminishes the reactivity of active sites (probably oxygen vacancies) where the dissolution process is initiated.