The adduct ions of two tetramolecular G-quadruplexes formed from the d(TGGGGT) and d(TTGGGGGT) single strands with a group of cationic porphyrins, with different charges and substituents, and one neutral porphyrin, were investigated by ESI-MS and ESI-MS/MS in the negative ion mode. Formation of Q + nNH(4)(+)+P(p+)-(z + n + p)H(+) adduct ions (where Q = quadruplex, n = number of quartets minus 1, P = porphyrin and p(+) = 0,1,2,3,4) indicates that the porphyrins are bound outside the quadruplexes providing an additional stabilization to those structures. The fragmentation pathways of the Q + nNH(4)(+)+P(p+)-(z + n + p)H(+) adduct ions depend on the number of positive charges (p(+)) of the porphyrins and on the overall complex charge (z(-)), but do not show a significant dependence on the type of the substituent groups in the porphyrins. Formation of the 'unfilled' ions Q + P(p+)-(z + p)H(+) predominates for porphyrins with a higher number of positive charges. Strand separation with the formation of T + P(p+)-(z-2 + p)H(+) and (SS-2H(+))(2-) ions, where T = d(TG(4)T) and d(T(2)G(5)T) and SS = d(TG(4)T) and d(T(2)G(5)T) is only observed for the complexes with a higher overall negative charge. Porphyrin loss with the formation of Q + nNH(4)(+)-(z + n)H(+) ions occurs predominantly for the neutral and monocharged porphyrins. The predominant formation of the 'unfilled' ions, Q + P(p+)-(z + n)H(+), for porphyrins with a higher number of charges shows that these porphyrins can prevent strand separation and preserve, at least partially, the quadruplex structure.