We study the elastic properties of homopolymer/hompolymer interfaces containing diblock copolymers by means of a theory of Gaussian fluctuations, The interfacial tension and the bending rigidity of the interface in the two-phase coexistence region are calculated from the power spectrum of capillary modes, Our theory shows that in the limiting case of a pure binary homopolymer mixture, while the interfacial tension increases monotonically with increasing chi N (where chi is the Flory-Huggins parameter and N is the homopolymer molecular weight) the bending rigidity does not. The bending rigidity increases rapidly at first for small values of chi N, but then decreases with further increase of chi N. In the presence of diblock copolymers, the interfacial tension always decreases with increasing diblock copolymer volume fraction at a given chi N. However, the bending rigidity can show either a decrease or an increase depending on chi N and the ratio gamma between the molecular weights of a diblock copolymer and that of a homopolymer. Our results for the surface pressure and the bending rigidity are further compared with results based on scaling arguments of wet polymer brushes.