The interfacial properties of lecithin layers in isooctane and in water-in-oil microemulsions were studied by electron paramagnetic resonance spectroscopy. The mobility of the spin probes 5- and 12-doxyl stearic acid (12-DSA) were monitored in both binary and tertiary lecithin systems. In the binary systems the two probes showed important differences caused by the relative positioning of the nitroxide group within the hydrophobic chains of lecithin. The concentration of lecithin also affected the mobility of the probe. In the lecithin microemulsions the presence of water and alcohol restored the probe mobility to some extent. Depending on the water content, the curvature of the lecithin layer was modified in a "hedgehog" way, i.e., when the size of the reverse micelles increased, the spacing among the hydrophobic lecithin tails decreased. The effect of the nature and concentration of the alcohol on the structure of the lecithin interface was also examined. The observed differences were mainly caused by the different partitioning of the alcohols among the various microdomains of the microemulsions. When an enzyme such as lipase was solubilized in the water core of the lecithin reverse micelles, the mobility of the 12-DSA was modified as a function of time. This was caused by the catalytic esterification of the fatty acid probe with propanol, a reaction that could be followed directly by monitoring the electron paramagnetic resonance spectra.