Phase transitions in poly(N-isopropylacrylamide) (PNIPAM) covalently labeled with a 2-naphthol were examined by steady-state and time-resolved spectroscopy. 2-Naphthol undergoes excited-state proton transfer (ESPT) at a rate that is dependent upon water concentration in aqueous solvent systems. Upon conversion to the phase-separated state in naphthol-labeled PNIPAM, the efficiency of excited-state proton transfer diminished, consistent with formation of a globular phase in which exposure of the probe molecules to the water phase was reduced.