The fluorescent properties of the cationic free base tetrakis (4-N-methylpyridyl)porphyrin (H(2)TMPyP(4)) in aqueous solution have been the subject of considerable discussion. Conclusions by various authors on the presence of home-aggregation of H(2)TMPyP in these solutions are contradictory. The present work reports spectroscopic data for three isomers of H(2)TMPyP(n) (n = 2, 3, or 4) at varying concentrations, solvent polarity, and temperature. H-1 NMR spectra of H(2)TMPyP(4) show no ground-state monomers below 10(-3) M in water. Fluorescence spectra of the three isomers in aqueous solutions indicate the absence of aggregates both in the ground and excited state. Fluorescence lifetimes of the three isomers both in solution as well as adsorbed on solid surfaces can be explained by taking into account their dependence on the steric hindrance for rotation of the pyridinium groups of the three isomers with respect to the porphyrin macrocycle. Molecular mechanics confirms a higher degree of steric hindrance of H(2)TMPyP(2) as compared to the two other isomers. From the experimental results it is concluded that the first excited singlet state S-1 of the porphyrin mixes with a nearby CT state slightly above this S-1 state. In this CT state an electron is transferred from the porphyrin macrocycle to the pyridinium group. The amount of S-1-CT mixing, responsible for the spectroscopic differences, is determined by the degree of coplanarity and resonance interaction of the porphyrin and the pyridinium pi-systems, and by the solvent polarity which determines the energy difference between the two states.