This paper reports on the first observation of phosphorescence emission in a thermoresponsive polymer in aqueous media. Random copolymerization of N-isopropylacrylamide, NIPAM, with bromostyrene, BrSTY, promotes efficient intersystem crossing into the triplet manifold, via the heavy atom effect, in an acenaphthylene label allowing generation of emission on a millisecond time scale. Luminescence techniques such as fluorescence time-resolved anisotropy, excited-state lifetime, and pyrene solubilization experiments have proven useful in probing the effects of heavy atom modification upon the thermoresponsive behavior of PNIPAM: BrSTY alters the hydrophobic/hydrophilic balance and serves to decrease the lower critical solution temperature, LCST, of the resultant polymer. The LCST is marked by a change in conformation of the macromolecule form an expanded water-swollen structure to a compact globule. The globular form of the BrsTY-modified polymer samples is so protective that phosphorescence emission is sustainable in aqueous solution at temperatures in excess of 40 degrees C.