Interfacial tension between polystyrene and polybutadiene is predicted by using a square gradient theory in conjunction with the Flory-Orvoll-Vrij equation-of-state expression for the free energy of mixing. The equation-of-state interaction parameter is determined by fitting the theory to the experimental cloud points. The square gradient coefficient is also calculated by the relation derived from a scattering function. The combined theory for interfacial tension predicts a magnitude and temperature dependence of interfacial tension although no adjustable parameter is used in calculating interfacial tension. The molecular weight dependence of interfacial tension is also predicted, and well described by a relation gamma=gamma(infinity)+CMn-z where z=2/3 for low molecular weight species and z=1 for higher molecular weight species.