Many drugs are both poorly water-soluble (hydrophobic) and lipophilic in nature and are therefore incorporated into surfactant micelles and monolayers. As the incorporation of drug into surfactant monolayers is expected to mirror its distribution in surfactant micelles, the study of drug distribution into surfactant monolayers should provide useful information about the nature and extent of drug solubilisation. However, to date very little work has been undertaken to determine the amount of drug incorporated and the extent of its penetration into surfactant monolayers. The present study will determine the amount and distribution of the testosterone enanthate into monolayers formed by a range of C12 surfactants of varying head group at the air/water surface. The study will provide invaluable information in understanding the preferred location of drug in surfactant micelles.