We present experimental results of a thermoreversible morphological transition between spheres and cylinders for polystyrene-block-polyisoprene diblock copolymers (SI) in dioctyl phthalate solutions. Morphologies were characterized using small-angle X-ray scattering (SAXS), and spherical and cylindrical states were found above and below the order-order transition (GOT) temperature, T-OOT, respectively. T-OOT increased with an increase of polymer concentration for the range covered in this study (higher than 70 wt % polymer concentration). The concentration dependence of T-OOT was found to be given by the relationship of(chi(eff)r(C))(OOT)/phi(p)r(C) = A + B/T-OOT, where (chi(eff)r(C))(OOT) denotes the critical value of the product of chi(eff) and r(C) at the cylinder-sphere GOT, chi(eff) is the effective interaction parameter between styrene and isoprene segments in the presence of solvent, r(C) is the reduced degree of polymerization of an entire block copolymer, and phi(p) is the volume fraction of the block copolymer in the solution. A and B are constants characterizing the temperature dependence of the segmental interaction parameter chi(SI) in bulk, where A = -0.0258 and B = 27.9 were evaluated by analyzing SAXS profiles from the disordered state. The critical values(chi(eff)r(C))(OOT) = 38.4 for the cylinder-sphere transition and 30.5 less than or equal to (chi(eff)r(C))(ODT) less than or equal to 32.3 for the order-disorder transition (ODT) were determined from our experimental result on the phi(p) dependence of T-OOT and that of T-ODT, respectively. These critical values were compared to the results of some theoretical studies. The SAXS measurements revealed thermoreversibility of the cylinder-sphere transition between 110 and 120 degrees C for an 80 wt % solution.