Aqueous micellar solutions and gels of the diblock copolymer E398B19 (E = oxyethylene, B = oxybutylene) have been investigated by rheology and small-angle neutron scattering (SANS) in the liquid, soft gel, and hard gel phases. Rheology experiments were used to confirm the microstructure and flow behavior of the system in the vicinity of the liquid-soft gel boundary previously located, as well as to identify a new liquid-hard gel transition at low temperatures. The dimensions of the micelles in the liquid phase were obtained from the micellar form factor described by a core/shell model. The parameters extracted from the modeling were in good agreement with previous results for the same system from light scattering. The micellar radius was found to decrease with temperature in the liquid phase, as a consequence of the contraction of the corona chains, caused by a decrease in solvent quality for poly(oxyethylene) at high temperature. The shape of the corona as a whole was preserved on increasing temperature, only undergoing a reduction in width. The equilibrium phase is fluid at volume fractions lower than the volume fraction for freezing of hard spheres, phi(f) = 0.494, in good agreement with a system of hard spheres. In the hard gel, the micelles are arranged in a bcc structure, as expected for micelles with lengthy E blocks.