Nineteen oxybutylene/oxyethylene/oxybutylene triblock copolymers [Bn/2EmBn/2, E = oxyethylene, OCH2CH2; B = oxybutylene, OCH2CH(CH2CH3)] were prepared and characterized. Twelve of the copolymers microphase-separated in the melt. Investigation of this microphase-separation behavior using small-angle X-ray scattering (SAXS) yielded values of the domain spacing (d-spacings) in the ordered phases and of the temperature of the order-disorder transition (T-ODT). In several cases the ordered phase structure was deduced from a combination of 1D and 2D SAXS and rheology. Values of TODT for the triblock copolymers were ca. 100 degrees lower than those for EmBn diblock copolymers of identical composition and chain length but 30 degrees C higher if compared with diblock copolymers of half the triblock length. Values of the d-spacings indicated that the triblock copolymers were 10% more stretched than corresponding diblock copolymers. Determination of the Flory-Huggins parameter (chi) for the diblock and triblock systems gave identical results. The experimental results are compared with the prediction of mean-field theory.