We report the synthesis and investigation of a new type of photoresponsive block copolymers (BCPs) They were designed to comprise two water-soluble polymers containing two different photoisomerizable moieties (either azobenzene and spiropyran or two different azobenzenes), with the two constituting blocks that, when separated, exhibit a lower critical solution temperature (LCST) in water and can shift their LCST in opposite directions upon photoisomerization (decrease of LCST for one polymer and increase for the other) A variety of such doubly photoresponsive BCPs were synthesized using either azobenzene- or spiropyran-containing poly(N,N-dimethylacrylamide) (PDMA), poly(N-isopropylacrylamide ) (PNIPAM) and poly[methoxydi(ethylene glycol) methacrylate] (PDEGMMA) Their thermal phase transition behaviors in aqueous solution before and after simultaneous photoreactions on the two blocks were investigated in comparison with their constituting blocks, by means of solution transmittance (turbidity) and variable-temperature H-1 NMR measurements The results show that BCPs displayed a single LCST whose shift upon two photoisomerizations appeared to be determined by the competing and opposing photoinduced effects on the two blocks Moreover, optically controlling the relative photoisomenzation degrees of trans azobenzene-to-cis azobenzene and spiropyran-to-merocyanine could be used to tune the LCST of BCP solution This study demonstrates the potential of exploring a more complex photoreaction scheme to optically control the solution properties of water-soluble thermosensitive BCPs (C) 2010 Wiley Periodicals, Inc J Polym Sci Part A Polym Chem 48 4055-4066, 2010