The phase diagram of coexisting liquid-liquid phase separation (LLPS) and crystallization in nearly isorefractive blends of statistical ethylene/hexane (PEH) and ethylene/butene (PEB) copolymers has been investigated. A variety of techniques that exploit crystallization-induced contrast, such as diffuse light scattering, small-angle light scattering, light transmission, atomic force microscopy, and differential scanning calorimetry, are used to identify the LLPS boundary, which is found to exhibit an upper critical solution temperature of 146 degreesC. The composition dependence of the LLPS boundary follows the prediction of Floly-Huggins theory for binary polymer mixtures. The equilibrium melting temperature of the blends decreases with increasing PEB concentration in the miscible phase, whereas it remains relatively constant at 127 degreesC within the LLPS coexistence region. Measurements of the LLPS boundary can be complicated by heterogeneity in the polymer microstructure.