Amphiphilic linear surfactants based on PEO and PPO triblock copolymers, known as Pluronics, exhibit a rich polymorphism and have been extensively studied for their applications in drug delivery and targeting. This work focuses on a promising and yet far less studied family of polymeric surfactants based on PEO and PPO known as Tetronics, which are four-armed (X-shaped) copolymers. Tetronics show a clear advantage over Pluronics because of their pH-responsiveness. In this proposal, we seek to establish the detailed structure of Tetronic micelles and in particular the mixtures of direct and reverse Tetronics which have been show to significantly enhance drug solubilisation, as a function of pH and temperature. This fundamental structural understanding is essential for the subsequent development of drug-carriers of Tetronic micelles with tailored release properties.