The lyotropic phase behavior of three poly(1,2-butadiene-b-ethylene oxide) diblock copolymers (PB-PEO) with different monomer volume fractions has been studied in two different ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]), across the complete concentration range. The ordered microstructures present in the solutions were characterized via small-angle X-ray scattering (SAXS). The phase diagrams for the PB-PEO/ ionic liquid solutions include regions corresponding to the classical copolymer microstructures: body-centered-cubic lattices of spheres, hexagonally ordered cylinders, and lamellae. Additionally, the phase diagrams also include wide regions of coexisting microstructures and regions apparently corresponding to a disordered network microstructure. The phase behavior of the PB-PEO copolymers in both ionic liquids was comparable to their previously reported aqueous solution behavior. The temperature dependence of the phase diagrams was very modest, indicative of a highly segregated system. The level of solvent selectivity was also investigated via cryogenic transmission electron microscopy (cryo-TEM) on dilute solutions. On the basis of the morphology of the dilute solution copolymer aggregate structures in the ionic liquid solvents, and on the structural length scales of the concentrated solutions, it was concluded that for PB-PEO [BMI][PF6] behaves as a more selective solvent than [EMI][TFSI].