Thermoelectric oxide-based multiphase systems gain synergistic properties from different materials. Therefore, multiphase systems based on a thermoelectric oxide, combined with both a polymeric phase (Matrimid) and a highly electrically conducting phase (Ag, carbon black) have been investigated. Compared to single-phase porous Ca3Co4O9, the resulting composite materials showed a decreased electrical conductivity while reaching a high Seebeck coefficient of up to 200 μV/K as well as a 4 times lower thermal conductivity. The strongly enhanced phonon scattering in the multiphase system resulting in low thermal conductivity is an especially interesting concept to design thermoelectric multiphase materials. Additionally, Ioffe plots are revitalized to compare the resulting power factor and thermal properties of the composite materials. The significantly low thermal conductivity due to the heteromaterial interfaces in the composite materials especially underlines the potential of multiphase systems as thermoelectric materials.