A new photoresponsive amphiphilic triblock copolymer, poly(pyrenylmethyl methacrylate)-block-polystyrene-block-poly(ethylene oxide) (PPy-b-PSt-b-PEO), was synthesized using atom-transfer radical polymerization. Formation of colloidal aggregates of the polymer was observed in solutions under controlled conditions due to the amphiphilic nature of the polymer. Irradiation of the polymer aggregates using UV light resulted in the photodissociation of 1-pyrenemethanol units from the polymer back-bone resulting in break-up of the aggregates mainly due to the hydrophilic nature of the residual polymer. The use of these polymer aggregates to trap hydrophobic fluorescent dyes in water and its controlled release on exposure to UV light has also been explored. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4448-4457, 2011