Mercury-dithizone complex both free and bonded to a polymeric system has been synthesized and used to measure the thermal transition of polystyrene, poly(vinyl acetate), and some other polymers. The thermal relaxation rate of the activated complex in dark has been found to be dependent on the free volume of a polymer matrix. The rate goes through a maximum above T-g of a polymer. A very sensitive method, based on thermal recovery of activated photochromic probe chromophore has been devised to measure the thermal transition of both single and multicomponent polymer matrices.