The antiseptic, antibacterial, and antifungal properties of the essential oil of Melalcuca alternifolia, commonly called tea tree oil (TTO), are well-known. This mixture of monoterpenes, sesquiterpenes, and their alcohols is utilized in many therapeutic and cosmetic applications, directly as essential oil or in different kinds of formulations, The terpinen-4-ol, which is present in the commercial essential oil samples in the minimum amount of 30 wt %, is recognized as the main active agent of TTO. Considering the good performance of lipid-based liquid crystalline phases as drug delivery systems, the monoolein/water system has been assessed for both terpinen-4-ol and TTO formulations. The phase behavior of the ternary systems has been investigated by means of optical microscopy and H-2 NMR quadrupolar splitting, C-13 NMR spin-lattice relaxation times and self-diffusion coefficients (measured by C-13 NMR PGSE technique) of the lipid matrix and the active agent have been determined to give insights on the dynamic and localization of the molecules in the different microstructures.