Water-in-oil (w/o) microemulsions, composed of virgin olive oil as the continuous oil phase, a mixture of lecithin and propanol as emulsifiers and water as the dispersed phase, have been successfully used as model biomimetic media to carry out enzymatic reactions that may naturally occur in olive oil. To evaluate the size of the dispersed polar domains as a function of water content, dynamic light scattering (DLS) measurements were carried out and radii in the range of 60-130 nm were observed. The polarity of the interior of these polar domains was studied by electron paramagnetic resonance (EPR) spectroscopy using a polar spin probe. Up to certain water content (1% w/w) both polarity and spin probe's mobility were affected by water content of the system and then remained unaltered. Oxidative enzymatic reactions using a native olive peroxidase (POD) also present in virgin olive oil, and horseradish peroxidase (HRP) were carried out in w/o microemulsions using 2,2'-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS) and gallic acid as substrates. Enzymatic activities were affected by the restricted environment of the nanoreactors and also the different polarity of the medium leading to comparable initial velocity values for both substrates. (C) 2010 Elsevier B.V. All rights reserved.