The concentration and temperature dependence of the structure of a poly(styrene-b-ethylene-alt-propylene) (PSPEP) diblock copolymer in squalane, a highly selective solvent for the PEP block, was investigated using small-angle X-ray scattering (SAXS). With decreasing concentration, between 100% and 10% by weight, a nearly monodisperse PSPEP (block molecular weights of 42 000 g/mol and 62 000 g/mol for PS and PEP, respectively) exhibited lamellae (LAM), hexagonally packed cylinders (HEX), body-centered cubic spheres (BCC) and disordered spherical micelles. The results show that squalane (C30H62) is essentially completely segregated into the PEP domains, and thus it behaves like a short PEP homopolymer. With increasing amounts of squalane the conformation of the PS block changes from stretched to nearly fully relaxed at the lowest polymer concentrations.