A preliminary investigation on the association behaviour of polyoxyethylene (20) sorbitan mono-oleate (Tween 80) in water and hexane was carried out at 30°C by constructing the phase equilibrium. The phases were separated by centrifugation. The birefringence was observed under cross polarizer and their consistency, texture and optical pattern were examined under polarizing microscope. From the phase equilibrium, several compositions were then selected to formulate O/W emulsions. Essential oil extracted from Kaffir lime was then incorporated into the emulsions to investigate their stability. Results showed that Tween 80 was readily solubilised in the mixture of water and hexane by forming association structures such as micro-emulsion and liquid crystalline phases. From the stability test, it was observed that the emulsions, where the compositions of water, hexane and Tween 80 that passed through the liquid crystalline region, were more stable.