Three derivatives of 1H,5H and 3H,5H-benzo[ij]quinolizin-5-one (BQZ1), previously prepared by chemical synthesis with the aim of obtaining furocoumarin analogs, have been studied. These are able to intercalate inside DNA and by subsequent irradiation with UVA light, to photoreact with DNA. Compound I (10-methoxy-7-methyl-1H,5H-benzo[ij]quinolizin-5-one) has a potentially photoreactive 2,3 double bond because of its conjugation with the pyridine ring of quinolinone, while compounds II (10-acetoxy-7-methyl-3H,5H-benzo[ij]quinolizin-5-one) and III (10-methoxy-7-methyl-3H,5H-benzo[ij]quinolizin-5-one) have a potentially photoreactive 1,2 double bond conjugated with the benzene ring of quinolinone. Compounds I and III, having a tricyclic planar structure, intercalate inside the DNA, while compound II cannot intercalate efficiently because of the steric hindrance of the acetoxy group in 10, lying outside the plane of the molecule and rotated by an angle of 77.6 degrees with respect to the tricyclic plane. The photoreaction of BQZ with DNA structure, as already known for psoralen and angelicin derivatives, consists of a [2 + 2] photocycloaddition reaction with the pyrimidine bases. The main photoadduct between the 2,3 double bond of I and the 5,6 double bond of thymine has been isolated and characterized by NMR, showing a cis-anti structure. Theoretical calculations, using AM1 Hamiltonian, have been carried out to describe the photocycloaddition reaction mechanism better. From a theoretical point of view, in the case of BQZ both the 1,2 or 2,3 double bonds and the 6,7 double bond may be involved in the [2 + 2] photocycloaddition. Spin densities and molecular orbital symmetries of compound I, in its triplet state, suggest that the 2,3 double bond interacts favorably with the 5,6 double bond of thymine moiety. On the contrary, the acetoxy substituent in position 10 of II seems to play a negative role in the DNA intercalation process.