Oxidative enzymatic reactions using horseradish peroxidase (HRP) were carried out in water-in-oil (w/o) microemulsions composed of olive oil/lecithin/1-propanol/water, a model biomimetic system. The substrates used (gallic acid, octyl gallate and 2,2'-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS)) have different hydrophobicities and possible locations in the microemulsion system. HRP reactivity with reference to substrate hydrophobicity and structural characteristics of the microemulsions is discussed. The nature of the enzyme microenvironments was examined using dynamic light scattering (DLS), differential scanning calorimetry (DSC) and diffusion NMR (DOSY) methodologies while the location of various enzymatic substrates in the microemulsion phase was assessed by solubility measurements and by taking pressure-area isotherms of mixed monolayers of the substrates with dipalmitoyl-phosphatidylcholine (DPPC), which is a major constituent of lecithin. In contrast to the bulk aqueous phase, in the severely restricted environment of the polar domains of the microemulsion HRP reacted faster with octyl gallate, a substrate that is solubilized at the lipid interfaces. HRP was deactivated in the olive oil microemulsions within a few hours, a phenomenon that has also been observed in other microemulsion systems.