Many drug molecules are both water-insoluble and lipophilic/slightly amphiphilic which limits their commercialisation and use by the patient. Solubilisation of water insoluble drug in surfactant micelles is an attractive means of increasing their apparent aqueous solubility thereby ensuring their delivery and ultimate use by patients. Surprisingly little is known about the relationship between surfactant and drug structure and the micelle's ability to solubilise drug. As a drug's incorporation into surfactant monolayers mirrors its distribution in surfactant micelles, the study of drug distribution into surfactant monolayers should provide useful information about drug solubilisation. This study aims to determine the level of incorporation and the extent of penetration of three lipophilic drugs, testosterone and two of its ethyl esters, into monolayers formed by sodium dodecyl-sulphate and N,N-dimethyldodecylammoniopropanesulfonate. The results study will be correlated with SANS studies determining the location of the drugs in the surfactant micelles and thereby aid a better understanding of the relationship between surfactant and drug structure and micelle solubilisation capacity.