We obtained and characterized some CdO thin films that show a single cubic rocksalt phase and a high free-electron concentration. SEM measurements confirmed the presence a grain structure with typical lateral dimensions of order 50–100 nm. Electrical characterization indicated that these CdO films behave as highly degenerate semiconductors, with a Fermi level well above the bottom of the conduction band. The carrier density is estimated to be about 7–8×1020 cm−3. The photocatalytic activity of the CdO thin films was investigated on the basis of the decolorizing efficiency towards the methyl orange dye. A first order kinetics was obtained. A small CdO surface area was effective for the total dye decomposition. All the present results were highly repeatable. The films studied in the present work have considerable potential for application as materials for water treatment.