Twenty-six poly(oxyethylene)-poly(oxybutylene) diblock copolymers were prepared by anionic polymerization. The phase behavior of these copolymers was studied near the order-disorder transition over the composition range from 0.21 to 0.84 poly(oxyethylene) volume fraction. Small-angle X-ray scattering was used to characterize phase transition temperatures and ordered state symmetries. Four distinct microstructures were observed : body-centered cubic, hexagonally packed cylinders (hex), lamellae (lam), and a bicontinuous cubic phase with symmetry, together with a less well-defined region of the phase diagram comprising either hexagonally perforated layers or biphasic hex and lam. The Flory-Huggins parameter as a function of temperature was estimated using both the mean-held approximation and a fluctuation correction to the mean-field theory. The experimental microphase behavior is compared with the exact mean-field phase diagram calculated using self-consistent-field theory.