Solubilisation of poorly-water soluble drug in surfactant micelles is an attractive means of increasing the apparent aqueous solubility of such drugs in order to administer the drug. Surprisingly little is known, however, about the relationship between surfactant and drug structure and the ability of the micelle to solubilise drug. As a drug’s distribution in a surfactant monolayer is related to its distribution in the corresponding micelles, the present study aims to investigate the location of testosterone propionate (TP) in an SDS monolayer at the oil-water interface using neutron reflectometry in combination with contrast variation. The data thus obtained will be compared with the corresponding data we recently obtained at the simpler air-water interface. The study will provide invaluable information of the location of TP in SDS micelles.