Top of pageAbstract A theory for the interfacial tension and interface thickness of demixed polymer mixtures and polymer solutions is presented on the basis of the theory of Cahn and Hilliard, taking into account the change in the dimension of polymer coil at the interface. The following results are obtained from the theory: (1) Near critical temperature Tc in a polymer mixture with the same chain length m (a symmetrical system), interfaical tension σ increases with m in the 1/2 power of m, whereas in a polymer solution, σ decreases with m in the—1/4 power, compared at the same reduced temperature T/Tc. On the other hand, the thickness of the interface near Tc increases with increasing m, proportional to the unperturbed chain dimension, i.e., to m1/2, in a symmetrical polymer system, and to the 1/4 power in a polymer solution. (2) In high polymer systems, a first order transition from a diffuse interface to a sharp interface is predicted. The transition temperature reduced by Tc increase with increasing m. The thickness of the interface at a low temperature does not greatly depend on m and is of the order of a segment length. Keywords: Interfacial Tension; Interface Thickness; Transition; Polymer; Non-Uniform System; Mixture; Solution; Critical Phenomena