Statistical amphiphilic copolymers of polymethacrylate having a lilial-derived acetal moiety as a hydrophobic side chain and poly(ethylene glycol) (PEG) as a hydrophilic side chain were synthesized with different unit ratios (3a (x:y = 100:0), 3b = 73:27), and 3c (x:), = 24:76)). In aqueous media the polymer 3c (x:y = 24:76) formed its micelles, whose particle diameter was ranged mainly from 10 to 220 run and averaged diameter was 35 nm. Polymer 3c in aqueous media exhibited aggregation-dissociation behavior which was dependent sharply and reversibly on temperature in the presence of NaCl. The aggregation of the polymer micelles inhibited hydrolysis of the acid-labile acetal side chain and release of the resulting filial, while its dissociation removed the inhibition. This responsive behavior by simply heating and cooling could control hydrolysis of the acid-labile acetal side chain and release of the resulting lilial.