The interaction and subsequent photoaddition reaction on UVA irradiation of 4.4'-dimethyl-5-methoxyangelicin and 4.4'-dimethyl-5-methoxypsoralen to twelve synthetic oligonucleotides was studied. A new approach was developed which allows: (a) accurate determination of the kinetics constant of the overall photoaddition process to each oligonucleotide; (b) identification of the different types of intercalation sites, individualized by a combination of two consecutive base pairs and the frequency of their appearance along the: oligomer sequences; and (c) the value of the kinetics constant relative to each type of site to be determined. This approach also makes it possible to highlight the influence of sequences flanking each site that participate in the photoaddition process. Thus, the possibility of discriminating which sites are active in the photoaddition reaction and their contribution to the overall process appears to be a useful tool Tot studying the correlation between furocoumarin photocombination with DNA and the consequent biological effects.