The phase behavior and structures of a poly(ethylene oxide-block-isoprene) diblock, having a volume fraction of poly(ethylene oxide), f(PEO), of 0.64, have been determined as a function of polymer concentration in water, and of temperature between 5 and 80 degreesC, using rheology and small/wide-angle X-ray scattering. Sol-gel boundaries were established by rheometry whereas small-angle X-ray scattering was employed to identify the lyotropic phases. For concentrations of diblock below 15 wt %, a liquid-like structure is obtained. Above 25 wt % copolymer in water, a lamellar morphology is observed which exhibits, for the pure sample, an order-disorder transition at 190 degreesC. Between these two concentrations a face-centered cubic (fcc) structure is adopted. As the concentration of copolymer is increased in the fcc regime, the size of the spheres increases before to transform directly into a lamellar morphology.