Periodically reversed (PR) electrolysis drastically reduces the durability of IrO 2Ta 2O 5/Ti electrodes for oxygen evolution compared to continuous anodic electrolysis, and the electrode's lifetime shortens with increasing cathodic current density. The reason for this anomalous deterioration induced by periodic cathodizing was examined in terms of the morphological and compositional changes in the IrO 2Ta 2O 5 coating during continuous cathodic electrolysis. The surface morphology after continuous cathodic electrolysis was quite different from those created by PR and continuous anodic electrolyses, accompanied by the appearance of round-shape IrO 2 particles. The compositional analysis of the oxide coating using EDX and XRF revealed that the cathodizing induced the selective consumption of tantalum oxide in the outermost layer. The mechanism governing deterioration during the PR electrolysis was discussed in terms of the comparison of the present results with those relating to the degradation observed during continuous anodic electrolysis obtained in a previous study.