The disordered and ordered states of a PEP-PEE diblock copolymer have been studied at different temperatures and pressures by small-angle neutron scattering, with the main goal to determine the relation between hydrodynamic pressure and the Flory-Huggins interaction parameter of a diblock copolymer melt. The structure factor has been analyzed in terms of the Fredrickson-Helfand theory, which gives the Flory-Huggins parameter, the spinodal temperature T-S, the order-disorder temperature T-ODT, and the order-order temperature T-OOT A decrease of T-ODT and T-S with increasing pressure of similar to 2 x 10(-2) K/bar is observed experimentally and is related to a decrease of the enthalpic term of the Flory-Huggins interaction parameter. No change of the T-OOT temperature with pressure was observed. The coil size as analyzed from the peak position of the structure factor shows stretching at lower temperatures. The compressibility of the coils changes with temperature and shows a maximum at the T-OOT temperature. Overall, diblock copolymers and binary homopolymer mixtures show distinct differences with respect to thermodynamic and conformational changes with varying temperature and pressure.