Oil-in-water microemulsions regions in systems of indometha-cin, isopropyl myristate, polysorbate 80, sorbitol and water have indicated an optimum polysorbate:sorbitol mass ratio for micro-emulsion formation of 1:2.5. The time-average light scattering technique results for this system have shown a linear increase of droplet size with an increase of oil content. An equation contained the partition coefficient of the drug between dispersed and continuous phase was used to estimate the permeation of the drug from microemulsions through a hydrophilic-lipophilic barrier. An increase of total polysorbate and sorbitol concentration from 32% to 44% w/w caused an increase of the permeability coefficient of indomethacin from 0.9 to 2.1 10−5cm/min.