Phase transitions in sphere-forming polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymer and its blends with polystyrene (PS) were investigated using oscillatory shear rheometry, transmission electron microscopy, and small-angle X-ray scattering (SAXS). For the investigation, a commercial grade of high-molecular-weight SIS triblock copolymer (Vector 4113, Dexco Polymers) and a low-molecular-weight SIS triblock copolymer (SIS-100) synthesized in our laboratory were used. It is found from oscillatory shear rheometry and SAXS that both Vector 4113 and SIS-100 undergo order-disorder transition (ODT) having a characteristic feature of lattice disordering/ordering transition (LDOT) of spheres, giving rise to a phase consisting of micelles with short-range liquidlike order at thermal equilibrium above ODT and demicellization/micellization transition (DMT), as a pseudo phase transition, giving rise to a micelle-free phase with only thermal composition fluctuations. The LDOT temperature (T-LDOT) of Vector 4113 is found to be ca. 205 degreesC, and the DMT temperature (T-DMT) is much higher than 220 degreesC, the highest experimental temperature employed to avoid thermal degradation/cross-linking reactions. The addition of various amounts of low-molecular-weight PS enabled us to estimate, via extrapolation, the T-DMT of Vector 4113 to be ca. 260 degreesC. On the other hand, SIS-100 is found to have T-LDOT of 85 degreesC and T-DMT of 105 degreesC. Transmission electron microscopy confirmed the existence of disordered micelles in both Vector 4113 and SIS-100 between T-LDOT and T-DMT. Phase diagrams for (SIS-100)/PS blends were constructed using T-LDOT, T-DMT, and cloud point measurements.