We have investigated the repeat distance and interface thickness, at 160 degrees C, of a poly(styrene-b-butyl methacrylate) (PS-b-PBMA) diblock copolymer of similar to 180 000 molecular weight by small-angle X-ray scattering (SAXS) and fluorescence resonance energy transfer ( FRET). We have found a lamellar period of 47 nm and an interface thickness of 5 nm. A simple, hyperbolic secant model of the junction distribution appeared to be sufficient to analyze the fluorescence decay data on the junction-labeled polymer containing different acceptor/donor ratios, but simulations based on a numerical self- consistent-field (NSCF) formalism also allowed us to find a range of approximately 0.017-0.018 for the Flory-Huggins interaction parameter (chi(FH), defined with reference to the monomeric volume of polystyrene) and a PBMA Kuhn length (b(PBMA)) of 0.65-0.67 nm. We note that earlier values of chi(FH) and b(PBMA) reported in the literature vary considerably. The NSCF computations suggest that even modest levels of conformational asymmetry perturb the block copolymer morphology. A weakness of our theoretical (NSCF) approach is the compressible nature of PS-b-PBMA. However, lattice cluster, equation of state (EOS), or other models that allow for compressibility have not yet been developed to the level of sophistication needed to predict block copolymer repeat distances or interface thicknesses. Indeed, as detailed in the Supporting Information, current EOS methods leave much to be desired even in predicting the phase transitions of lower molecular weight PS-b-PBMA samples.