Anionic meso-tetrakis(4-sulfonatophenyl)porphine (TSPP) interacts with poly(amido) amine (PAMAM) dendrimers of generations 2.0 and 4.0 in aqueous solution to form several dendrimer-associated species, which depend on the relative concentrations of dendrimer to porphyrin (D/P equivalent to [PAMAM]/[TSPP]). At D/P ratios above the isoelectric point of the charge balance between the dendrimer and porphyrin, only two spectroscopic species were detected. An equilibrium model assuming a dendrimer-induced TSPP H-dimer coexisting with dendrimer-associated TSPP monomer afforded a good description of the experimental data, giving equilibrium constants (K-d) for the dimer dissociation of 0.78 and 2.67 for generations 2.0 and 4.0 dendrimers, respectively. The decomposition of the Soret band afforded TSPP H-aggregates' spectra with bandwidth values in the range 910-1210 cm(-1), clearly larger than the monomer bandwidth around 711-800 cm(-1). The fluorescence decays are nearly exponential with a long decay time component, which varies from 10 to 12 ns depending on the D/P ratio. However, at pH 2, there are striking differences between generations 2.0 and 4.0, which should be related to a more hydrophobic environment provided by the latter one. Time-resolved fluorescence anisotropy gave rotational times for the porphyrin-dendrimer complex from which hydrodynamic radii of approximately 14 and 21 Angstrom, similar to those of generations 2.0 and 4.0 PAMAM dendrimers, were retrieved.