The effects of molecular weight and microdomain orientation on the ionic conductivities of polystyrene-b-methyl methacrylate) diblock copolymer/1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide (SM/IL) mixtures are assessed through complementary experimental and theoretical techniques. Small-angle Xray scattering revealed that SM/IL mixtures have anisotropic lamellar morphologies, preferentially oriented parallel to the casting substrate. A method for quantifying the morphology factor, or microdomain orientation within the Sax-Ottino model, using 2-D SAXS data is presented and applied to SM/IL mixtures. Ionic conductivity increases by up to an order of magnitude with a 2-fold increase in molecular weight,, even when accounting for the morphology type, composition, microdomain orientation, and PMMA/IL glass transition temperature. Self-consistent field theory calculations predict a nonuniform solvent distribution in PMMA/IL, microdomains, suggesting that polymer mobility and ion transport are reduced near PS-PMMA microdomain interfaces. Thus, the increase in ionic conductivity with increasing block copolymer molecular weight is associated with having fewer PS-PMMA/IL interfaces per unit volume.