An amperometric enzyme electrode for organic peroxides, based on the incorporation of horseradish peroxidase (HRP) into a carbon paste matrix, is reported. The electrode responds rapidly to low (micromolar) concentrations of such peroxides, in accordance to the following sensitivity trend: butanone peroxide > tert-butyl peroxibenzoate > cumene hydroperoxides > triphenylmethyl hydroperoxide > tert-butyl hydroperoxide > tert-amyl perbenzoate > tertbutyl peroxiacetate. The effect of various operational parameters is explored for optimum analytical performance. The dynamic properties of this electrode are exploited for detection in flow injection systems. Work in mixed nonaqueous-aqueous media allows the quantitation of highly hydrophobic peroxides and extension of the linear range. Similar organic peroxide detection is reported for the incorporation of HRP within a rigid graphite-epoxy matrix, and for a tissue (horseradish-root) modified electrode. Applicability of the HRP electrode for assays of drinking water is illustrated and prospects for its utility in environmental work are discussed. Analogous measurements using another (fungal) peroxidase are reported.