We investigated the order-order and order-disorder transitions in a solution consisting of polystyrene-block-polyisoprene-block-polystyrene copolymer (Vector 4111 having weight-average molecular weights of 26 x 10(3) and 117 x 10(3) for polystyrene and polyisoprene block chains, respectively) and dioctyl phthalate (DOP), using small-angle X-ray scattering (SAXS), rheology, and transmission electron microscopy (TEM). SAXS results obtained in a heating process show (a) that, in the 85.5/14.5 weight fraction Vector 4111/DOP solution, (1) an order-order transition (OOT) takes place at 130-140 degrees C, (2) a lattice-disordering transition (LDT) takes place at 150-155 degrees C, and (3) an order-disorder transition (ODT) takes place at ca. 205 degrees C and (b) that, in the 73.2/26.8 Vector 4111/DOP solution, (1) an OOT takes place at 80-90 degrees C, (2) no LDT takes place because the structure formed after the OOT contains a considerable amount of lattice distortion, and (3) an ODT takes place at ca. 130 degrees C. Both SAXS and TEM results show that, in the 85.5/14.5 Vector 4111/DOP solution, hexagonally-packed cylindrical microdomains of polystyrene transform into spherical microdomains at the OOT temperature (T-OOT) Comparison of SAXS results with rheological results indicates that the addition of 14.5 wt % DOP lowered both the T-ODT and the lattice-disordering transition temperature of bulk Vector 4111 by ca. 50 degrees C. By assuming that DOP strictly plays the role of diluent, we estimated the ODT temperature (T-ODT) Of bulk Vector 4111 to be ca. 280 degrees C, which supports the conclusion drawn in our previous study that the T(ODT)of bulk Vector 4111 is much higher than 220 degrees C.