A photochemical and photophysical study on three quinolinium iodides, free and complexed with DNA, was carried out by stationary and pulsed techniques. The comparison with the behaviour of pyridinium analogues pointed to a reduction in the photoisomerization yield of these compounds in favour of fluorescence and internal conversion. These quinolinium salts showed a good affinity towards salmon DNA with association constants ≈10(4) M(-1) and a different photobehaviour when involved in the formation of ligand-DNA complexes. The drastic reduction of photoisomerization observed under excitation of 1/3-DNA complexes and accompanied by a huge increase of the fluorescence quantum yields was connected to interaction modes favouring intercalation proposed for these planar quinolinium derivatives.