A solvent cast film of polyacryonitrile (PAN) incorporating the fluorescence probe p-N,N-dialkylaminobenzilidienemalononitrile (DABN) was studied for its fluorescence over the temperature range 30-180 degrees C. It was observed that the thermal effects on the fluorescence of the dye molecule are dependent on the polymer matrix surrounding it. The rate of decrease in fluorescence of DABN shows a remarkable change after 110 degrees C. This temperature corresponds to the glass-transition temperature (T-g) of the polymer PAN. The observed phenomenon is explained in terms of the change in free volume of the polymer when the polymer changes from a glassy state to a rubbery state at its T-g. Thus the fluorescence probe technique finds useful application for conducting thermodynamic studies of polymers.