An advanced theoretical model for the structure of the interface between two immiscible homopolymers is presented. The theory is based on a combination of the Flory-Huggins model and the square-gradient approach. We first analyze the effect of molecular weights of the components on the interfacial tension in the limit of high molecular weight (chi N >> 1). A general approximate expression for the interfacial free energy of the system is derived, the expression being valid for both limits of narrow and broad interfaces. The interfacial tension for a system of two immiscible homopolymers is calculated as a function of the homopolymer molecular weights and the Flory chi-parameter. The results are compared with the predictions of other theories and also with experimental data.