Fluorescence from the fluorescein molecule has been used to study of phase transition processes in five poly(vinyl alcohol) (PVA) hydrogels in the temperature range 75-350 K. These hydrogels were prepared with PVA with molecular weight = 124,000-186,000, 99+% hydrolyzed, by repeated freezing-thawing cycles in five compositions: 9.9, 11.1, 13.3, 16.5, and 20.9 PVA w/w %. Plots of total fluorescence intensity versus temperature, normalized with the fluorescence signal at 75 K, and of the emission wavelength maximum versus temperature indicated that the dynamic of the guest deactivation was dependent on the hydrogel thermal transitions. These thermal transition processes were compared to three model systems: uranine (fluorescein di-sodium salt) in water, fluorescein in water/ethanol (85%/15% v/v), and fluorescein/PVA homopolymer. Small spectral shifts were obtained for these model systems over the entire temperature range attributed to the hot band effect. Nevertheless, larger spectral shifts were obtained for hydrogels, indicating that shifts of the chemical dissociation equilibrium was occurring in this last case. Slope changes of both intensity and wavelength maximum plots versus temperature occurred at the same temperature values, and we attributed the onset of the hydrogel thermal transition as the sol-gel transition occurring at T congruent to 370 K; at 320 K we observed the glass transition temperature of PVA plasticized by water molecules; at T congruent to 240 +/- 5 K we attributed the melting point of water molecules bonded to the PVA chains (freezable bond molecules); at T congruent to 270 +/- 5 K we observed the melting point of free water molecules (bulk water); and, at 135-145 K the water glass transition. (C) 2000 John Wiley & Sons, Inc.