Pyrcnc probc and TEM have been employed to investigate the behavior of a polystyrenc-b-poly(acrylic aeid) (PS-b-PAA) copolymer in aqueous solution. A significant annealing temperature effect on the I1/I3 ratio of pyrene was observed and was interpreted in terms ofthe morphological change ofmicellar cores. Anncaling at a temperature higher than the glass transition temperature (Tg) of PS leads to densification ofthe hydrophobic core.