Density functional calculations have been carried out for several Ni porphyrinates representative of the most common form in which this metal is present in crude oil. A detailed study of the molecular and electronic structure is presented including the description of the low lying electronic states leading to the well known Q and Soret bands in the optical spectra. The reliability of the present study is confirmed by comparison to experimental data. It is found that presence of different substituents does not largely affect the electronic structure of the nickel porphyrin moiety including the energy excitation required to promote these molecules to the low lying electronic states. It is proposed that these promoted Ni-porphyrin derivatives can participate in poisoning catalysts like zeolites and metallic sulfides. However, the need for electronic promotion in Ni-porphyrinate derivatives indicates that this family of compounds is likely to be less active as catalyst poison than other metals like V, in agreement with experimental evidence. (C) 2004 Elsevier B.V. All rights reserved.