The phase behavior of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymers of various molecular weights has been studied by using small-angle X-ray scattering (SAXS) and depolarized light scattering (DLS). The empirical Flory χ, determined from scattering intensity profiles for a fully disordered PS-b-PMMA copolymer, was shown to behave differently depending on temperature range. χ was described mostly by enthalpic contribution at higher temperatures, but a dominant entropic contribution appeared in χ at lower temperatures. The order−disorder transition (ODT) temperatures for the series of copolymers with the controlled molecular weights were directly measured through SAXS and DLS. The resultant ODTs were then compared with a compressible random-phase approximation theory to determine cross interactions between block components. It was found that effective χ from theory is also mostly described by enthalpic contribution, which yields a moderate change in ODT upon the increase of copolymer molecu...