The cathodic properties are discussed of a ZrO2 layer formed on Zr by thermal treatment and by anodic oxidn. The electrosynthesis of ammonium peroxydisulfate (2SO42- .dblarw. S2O82- + 2e-) for which the cathodic redn. of persulfate to sulfate is the undesirable reaction is chosen as a typical industrial reaction to study the behavior of ZrO2 coated Zr cathodes towards the persulfate redn. and hydrogen evolution reactions. [on SciFinder (R)]