The anodic oxidation of lutetium diphthalocyanine films on tin oxide in was investigated by a galvanostatic transient technique. The reaction was monitored by simultaneous measurement of the optical transmission. At current densities of 0.3–6 mA/cm2, the kinetics was controlled by an ionic space charge in the red oxidation product. A dielectric constant of 10 was estimated for this material, using the solid‐state mobility of the chloride ion determined in a previous moving‐boundary study.