Two kinds of transitions involving a change in molecular assembly, lattice disordering/ordering transition (LDOT) and demicellization/micellization transition (DMT), in highly asymmetric polystyrene-bloch-polyisoprene (SI diblock) copolymers were investigated using oscillatory shear rheometry, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). For the study, three highly asymmetric SI diblock copolymers having the volume fractions of polystyrene (PS) block of 0.11, 0.13, and 0.16, respectively, were synthesized using anionic polymerization. In highly asymmetric block copolymer, LDOT is defined as a transition where the long-range order of microdomains is lost during heating, giving rise to a disordered arrangement of spheres with short-range liquidlike order (termed disordered spheres or micelles), and DMT is defined as a transition where all microdomains disappear during heating and are transformed into the micelle-free homogeneous state in which the component polymers are mixed on a molecular level and only thermally induced composition fluctuations may exist DMT may be regarded as a pseudo-phase transition, involving effectively a finite number of molecules and hence occurring over a finite range (possibly a narrow range) of temperatures. It was found that the three highly asymmetric SI diblock copolymers underwent, during heating, first lattice disordering and then demicellization transitions. The values of LDOT temperature (T-LDOT) and DMT temperature (T-DMT) for the highly asymmetric SI diblock copolymers were determined by careful analysis of SAXS results, and the value T-DMT determined by oscillatory shear rheometry was found in good agreement with SAXS results. The LDOT determined from SAXS was confirmed by TEM, showing the existence of disordered spheres at thermal equilibrium at T-LDOT less than or equal to T less than or equal to T-DMT.