The order-to-disorder transition (ODT) in a concentrated solution of a polystyrene-polyisoprene diblock copolymer (molecular weights of the polystyrene and polyisoprene blocks are 1.2 × 104 and 1.9 × 104, respectively) in bis(2-ethylhexyl) phthalate is identified by a discontinuous decrease in the static birefringence of the sample upon heating. The location of the ODT is confirmed by rheological measurements; both the storage and loss moduli of the solution show a sharp decrease at the same temperature. Solutions with concentrations ranging from 48 to 61 wt % polymer were examined with the birefringence technique. The measured ODT temperatures for these solutions range from 27 to 178 °C. Small-angle X-ray scattering measurements at temperatures well below the ODT indicate that the ordered solutions have a lamellar microstructure.