Abstract The phototoxicity of two new porphyrin photosensitizers, diarginine diprotoporphyrinate (PP(Arg)2) and N,N-diphenylalanyl protoporphyrin (PP(Phe)2), and the synergistic effect of 5-methoxyposralen (5-MOP) have been studied in comparison with that of protoporphyrin IX (PPIX). Under ultraviolet-A (UV-A) irradiation (λ = 365 nm), the phototoxicity of the porphyrins toward cultured human fibroblasts and keratinocytes decreases in the order: PPIX > PP(Arg)2 > PP(Phe)2. A synergistic effect of 5-MOP on the phototoxicity of PPIX, PP(Arg)2 and PP(Phe)2 has been observed. The combination of PPIX, PP(Arg)2 and PP(Phe)2 with 0.1–0.5 μM 5-MOP significantly potentiates the phototoxicity of the three porphyrins. The most effective potentiation was observed with the water-soluble PP(Arg)2 and 5-MOP concentrations lower than 0.75 μM. Above this 5-MOP concentration this potentiation is abolished. The intracellular concentration of PPIX and PP(Phe)2 is independent of the presence of 5-MOP. On the other hand, the intracellular content of PP(Arg)2 is decreased in a concentration-dependent manner by the psoralen. Illumination with red light, not absorbed by 5-MOP, leads to a weak potentiation of the PP(Arg)2 phototoxic effect in the presence of 5-MOP, suggesting that dark interaction of 5-MOP with cell membranes aggravated by porphyrin photosensitization is involved in the observed phenomena. The results are tentatively explained by differences in hydrophobicity and molecular structures of the examined photosensitizers. PPIX, which is barely soluble in water, has a significantly higher affinity for cell membranes and simultaneously exerts a stronger phototoxic effect than PP(Arg)2 whose solubility in water is high. On the other hand, the weak phototoxicity of PP(Phe)2 could be explained by the steric hindrance brought by the phenylalanyl substituents on the pyrrole ring. The loss in the PP(Arg)2 cell content probably explains the inhibition of the synergistic effect of 5-MOP on the PP(Arg)2 phototoxicity at high 5-MOP concentration. This study suggests that PP(Arg)2 in combination with 5-MOP might reveal a strong phototoxic effect when applied to skin cancer treatment.