The aim of the study is to use SANS in combination with contrast variation to determine the effect of solubilising the lipophilic drug, testosterone propionate, in micelles formed by the ionic surfactants, sodium dodecyl sulphate and dodecyl trimethylammonium bromide. This study is part of a larger study aimed at understanding the factors influencing the amount of drug solubilised in micelles as well as the influence of drug solubilisation on the size and shape of micellar aggregates.