A diblock copolymer of styrene and butyl methacrylate with a narrow molecular weight distribution [P(S-b-BMA)] containing 1200 monomeric units of S and 1250 of BMA was studied in 2-propanol (2-POH), which is a nonsolvent for the S blocks and a θ solvent (UCST) for the BMA blocks. The phase diagram of the present system is almost indistinguishable from that of 2-POH/PBMA if the molecular weight of the homopolymer is identical with that of the BMA block. The comparison of the intrinsic viscosities [η] of p(S-b-BMA) and PBMA in 2-POH and of their variation with temperature demonstrates that [η] P(S-b-BMA) reflects the presence of micelles rather than unimers