Using equations for the interfacial properties for a two-phase multicomponent system, we present a new model for the interfacial tension and bending rigidity for liquid-liquid interfaces between semidilute polymer phases. Using this model, we calculate the interfacial thickness and the bending rigidity for two different gelatin/dextran systems and a gelatin/gum arabic system using experimentally determined values for the interfacial tension. The bending rigidity of such systems has been unaccessible experimentally until now. For the gelatin/dextran systems, which are both near-critical, the interfacial thickness is very large (1000 nm) close to the critical point, where the interfacial tension is very low. Further from the critical point, the interfacial thickness decreases to a value in the order of the size of the biopolymers (100 nm). For the gelatin/gum arabic system, which is off-critical, we found the interfacial thickness to be constant, in the order of the size of the biopolymers. For the gelatin/dextran systems, the scaling relation between the interfacial tension and the interfacial thickness was investigated. The exponents were found to be approximately 1.7 for the two systems, which is in agreement with the exponent 2 of the scaling relation gamma similar to 1/xi(2). The accompanying bending rigidities for these near-critical gelatin/dextran systems were found to be approximately constant, with a value of 500 k(b)T. The bending rigidity for the gelatin/gum arabic system, which is off-critical, was in the order of 25 k(b)T. These high values for both the interfacial thickness and the bending rigidity for the near-critical systems may be of significance for interface-related phenomena in aqueous phase-separated biopolymer mixtures, in particular in cases where the bending contributions dominate the stretching contributions to the interfacial energy.