A combined experimental and computational study of the photocatalytic activity of titanium dioxide (TiO2) for the degradation of nitrogen(II) oxide (NO) and acetaldehyde is reported. We employ five different TiO2 photocatalysts including pure anatase and rutile samples. The experimental photonic efficiencies indicate that, under visible irradiation, the samples containing pure TiO2 are active for the decomposition of NO but inactive for the acetaldehyde degradation. This is in accordance with our theoretical predictions, which reveal the presence of weak absorption bands in the visible region of the absorption spectra of the TiO2–NO complexes. We demonstrate that these bands originate from charge-transfer excitations between the pollutant and the substrate. Although a ligand-to-metal charge-transfer process is expected to predominate, we find a competing mechanism in which one electron is promoted from the valence band of the semiconductor to the virtual π* states of NO. Both experimental and theoretical results show an enhanced vis-activity of anatase TiO2 compared to rutile. We observe from the theoretical simulations the formation of reactive monocoordinated oxygen atoms at the anatase (001) surface for moderately low concentrations of the contaminant. Based on these findings, a new mechanism for the photo-oxidation of NO is proposed.