A red/green anodic‐reaction boundary was propagated in a lutetium diphthalocyanine film by application of a constant current through a metallic and an electrolytic contact. From the boundary velocity and the electric field in the red phase, the oxidation product was found to contain a charge carrier with a concentration approximately twice that of the dye molecules and a mobility of . These results and the charge compensation requirement of the boundary process suggested that the red oxidation product was a solid anion conductor. The estimated bulk resistivity of the red material at room temperature was 1600 Ω‐cm.