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Premature color formation and fogging in composites used in two sheet thermographic imaging systems are prevented by providing a polymeric barrier coating between the two interactive layers. The barrier coat serves to keep the volatilizable organic acid in the donor sheet from diffusing into the dye precursor receptor sheet during storage or transport, but permits the desired diffusion at thermographic imaging temperatures. Suitable barrier coat layers, which must be acid-resistant, include chlorinated rubber, chlorinated polypropylene, styrene-acrylonitrile copolymer, chlorinated paraffins, polystyrene and vinyl chloride-vinyl acetate copolymer.