Absorption measurements and electron microprobe analysis are used to determine the density of Cu1+and Cu2+ions in a photorefractive KTa1−xNbxO3:Cu,V sample. Photorefractive measurements are made over a wide range of Cu1+and Cu2+concentrations, altered by a series of oxidation and reduction treatments. Diffraction efficiencies were varied by over an order of magnitude while erasure rates were varied by over two orders of magnitude.