Metal passivation and passive film breakdown in aqueous systems are complex processes. Zirconium passive films grown in basic solutions containing buffer, chloride and buffer; and chloride, sulfate and buffer have been examined with particular attention to the passive and transpassive regions using cyclic voltammetry (CV). Mechanically polished zirconium electrodes have been oxidized anodically in pH 8 buffer solutions containing 0.021 M chloride, with varying sulfate concentration. The CV curves show that zirconium forms passive layers under these conditions, which are more stable with increasing sulfate concentration. A model is proposed to explain the passive to transpassive transition taking oxygen evolution, crystallization and ohmic behavior into account.