Attemps have been made to establish the interrelations between composition distribution and melting endotherms of crystalline copolymers. Several basic equations have been obtained theoretically from the thermodynamics of copolymer crystals and its melt

and parameters of these equations have been measured empirically in the case of FEP copolymers. On the basis of these relations the melting endotherms can be converted to the composition distributions. Some preliminary results on composition distribution of FEP copolymers were given. It is founded that thecopolymers with broad distributions have a poor stress-cracking resistance.