Malachite green (MG) is a type of leuco dye that changes to a cationic form upon UV irradiation, and returns to a nonionic form in the dark. A random copolymer (p(NIPAM/SMG)) composed of N-isopropylacylamide (NIPAM) and a small amount of MG-bearing monomer (SMG) was prepared. Monitoring the absorbance of an aqueous solution of p(NIPAM/SMG) at 620nm revealed that absorbance increased upon UV irradiation due to formation of cationic MG. The lower critical solution temperature (LCST) of p(NIPAM/SMG) was shifted to a higher temperature upon UV irradiation compared to that in the dark.