The lower valent cations were homogeneously doped in p-type oxide films. The prepared doped oxide systems were Mg-doped alpha-Cr2O3, Ni-doped alpha-Cr2O3, and Li-doped NiO. The Mg doping induced a homogeneous mixed valence state of Cr3+ and Cr6+ in the Cr2O3 film, reducing the Mg dopant to the metallic valence state in the Cr2O3 lattice, which is completely different from the powder or single crystalline system. The mixed valence state formation gave rise to a marked chromism. The mixed valence formation and perfect reduction of the lower valent dopants were evidenced by X-ray photoelectron spectroscopy, electrical conductivity change, and optical absorption. Ni-doped alpha-Cr2O3 and Li-doped NiO showed similar results. The reduction kinetics of the mixed valence state in the p-type oxide lattice with the aid of the electrical conductivity measurement suggested that the rate-determining step is not the diffusion of oxygen.