Microemulsions are shown to extend the scope of enzymic amperometric assays towards both hydrophobic and hydrophilic compounds. This possibility is illustrated for amperometric monitoring of organic peroxides in oil-in-water emulsions containing horseradish peroxidase. Enhanced sensitivity (compared with work in pure aqueous solution) is observed also for water-soluble analytes, and is attributed to changes in the local substrate concentration. The effect of the emulsion structure on the biocatalytic activity and analytical performance is explored. Both hydrophobic and hydrophilic mediators can be used. Amperometry can also be used for elucidating biocatalytic conversions in microemulsions. Future prospects are discussed.