It has been shown that individual components of crude oil and their narrower fractions with respect to group and functional composition and molecular weight distribution, and also certain synthetic compounds modelling the structural fragments of petroleum components, change the spectral characteristics of nickel and vanadyl etioporphyrins (models of petroleum metal porphyrins) considerably. The greatest distortions in the analytical signal of nickel and vanadyl etioporphyrins are observed in the near-UV region of the absorption spectrum. Of the substances studied, the maximum spectral effect was observed in the presence of concentrates of high-molecular weight petroleum nitrogen bases, carbazole, phenanthrene and anthracene. The intensity of the analytical signal in this case falls almost to zero, The dependence of the distortion of the analytical signal on the quantity of the addition and the contact time is pointed out, The results obtained indicate the possibility not only of additive, but also of multiplicative hindrances in the quantitative analysis of petroleum metal porphyrins.