The present study has consisted to the disinfection of water by photosensitization with Bergamot essential oil. The results obtained show negligible inhibition of fecal coliforms in untreated water exposed to light. In contrast, for treated water exposed to light, complete inhibition of fecal coliforms was observed after one hour of exposure. In treated water kept in the dark, no inhibition of fecal coliforms was observed from the beginning to the end of the experiments. The activity observed in this oil is due to 5-methoxypsoralen (MOP-5). In the presence of light, this molecule is excited (1MOP-5*). The singlet excited form undergoes an intersystem crossing (transition from the singlet to the triplet state); from the triplet state, the 5-methoxypsoralen (3MOP-5*) transfers its energy to triplet oxygen (3O2). The triplet oxygen undergoes a transition from a triplet to a singlet state (1O2*). The singlet oxygen generated initiates a series of reactions with biomolecules, particularly the nitrogenous bases of DNA. These photo-oxidation reactions lead to the destruction of microorganisms.