Micelle formation and decomposition were investigated by static and dynamic light scattering for the diblock copolymer poly(alpha-methylstyrene)-block-poly(vinyl-p-phenethyl alcohol) (P alpha MS-b-PVPA) with asymmetric block composition [P alpha MS]/[PVPA] = 90/10 (in molar mass) in mixed solvents, one component solvent, m-chlorobenzyl chloride (CBC), being selective for the P alpha MS block and the other, m-chlorobenzyl alcohol (CBA), being so for the PVPA block. Core-corona type micelles were formed in both pure solvents : micelles in CBA were dense star-shaped ’crew-cut’ ones having a large swollen core with a thin corona layer, and those in CBC were star-shaped ones having a small core with long corona chains. Micelle decomposition was observed in both CBA- and CBC-rich solvents within a narrow range of solvent composition as the other component solvent was being added to a pure solvent. In the case of micelles formed in CBA-rich solvents, micellization could be induced by decreasing the temperature, while the micelles in CBC-rich solvents could not be decomposed in the ordinary temperature range. The temperature-induced micellization transition at higher polymer concentrations took place with no appreciable increase of micelle fraction but primarily with an increase of association number. On the other hand, in the transition region at a lower concentration, change of micelle fraction dominated over variation of association number. From these results, the following were suggested : the critical micelle concentration cmc* near the critical micelle temperature was located at a relatively high concentration in CBA-rich solvents, while the micelles formed in CBC-rich solvents were more stable having lower cmc*.