We have prepared temperature sensitive poly(N-isopropylacrylamide) [poly(NIPA)], and poly(NIPA)-acrylic acid copolymer [poly(NIPA-co-AA)] microspheres by dispersion polymerization and entrapped them in a poly(vinyl alcohol) hydrogel for optical sensing applications. Microsphere swelling causes the microsphere refractive index to decrease, bringing it closer to the refractive index of the hydrogel. This results in a turbidity change, which was measured by a miniature fiber optic spectrometer, Ocean Optics SD 1000. The phase transition temperature of poly(NIPA) microspheres in the membrane was consistent with those of poly(NIPA) in aqueous solutions. Because deprotonation of the acrylic acid causes a large change in the swelling properties of these microspheres, these membranes are sensitive to pH. Preliminary data were also presented to illustrate that complexation of the deprotonated acrylic acid units by Pb(II) and Cu(II) affects the phase transition. A small change in the affinity of the comonomer for the solvent has a large effect on swelling by shifting the phase transition temperature. This approach is attractive for developing polymer swelling based sensors for applications that involve a constant temperature or corrected temperature fluctuations. (C) 2004 Elsevier B.V. All rights reserved.