The frequency-dependent microwave complex permittivity of composites based on an Al2O3-CuO system is investigated at room temperature. The composites are formed by solution infusion of copper precursors into a porous Al2O3 matrix, followed by thermal decomposition to copper oxides and localized formation of copper aluminate compounds, and finally, H-2 firing. The material exhibits a strong amplitude, with relatively narrowband dielectric resonance in the microwave frequency regime at intermediate levels of mass gain, which is reminiscent of a plasmon resonance. Large mass gains cause negative permittivity behavior below 6 GHz, consistent with a collisional Drude model of a semiconducting oxide phase.