A series of carbon‐coated, nitrogen‐doped titanium dioxide photocatalysts was produced and characterized. N‐doped TiO 2 powder samples were prepared using a sol‐gel method and subsequently used for making stable doped‐TiO 2 thin films on glass substrates. Carbon layers were coated on the films by a thermal decomposition method using catechol. Diffuse reflection spectra and Mott‐Schottky analyses of the samples proved that nitrogen doping and carbon coating can slightly lower the band gap of TiO 2 , broaden its absorption to visible light and enhance its n‐type character. According to photocatalytic tests against model contaminants, carbon‐coated nitrogen‐doped TiO 2 films have better performance than simple TiO 2 on the degradation of Rhodamine B dye molecules, but are poorly effective for degrading 4‐chlorophenol molecules. Several possible explanations are proposed for this result, supported by scavenging experiments. This reveals the importance of a broad substrate scope when assessing new photocatalytic materials for water treatment, something which is often overlooked in many literature studies.