There has been a controversy about the self-aggregation of 5,10,15,20-tetrakis[4-(N-methyl)pyridinium]-porphyrin (TMPyP(4)) in water. In order to make clear whether TMPyP(4) forms a dimer in water, H-1 NMR and UV-vis spectroscopic studies of 5-phenyl-10,15,20-tris[4-(N-methyl)pyridinium]porphyrin (TriMPyP), 5,10-diphenyl-15,20-bis[4-(N-methyl)pyridinium]porphyrin (5,10-DiMPyP), and 5,15-diphenyl-10,20-bis[4(N-methyl)pyridinium]porphyrin (5,15-DiMPyP) have been carried out. The H-1 NMR spectra indicate the formation of the self-aggregates of these phenylpyridiniumporphyrins in D2O when the porphyrin concentration is 1 x 10(-3) M. Comparison of the H-1 NMR spectra or TMPyP(4) with those of TriMPyP and DiMPyPs clearly exhibits that TMPyP(4) does not form its dimer in D2O. Broadening of the signal due to the beta-pyrrole protons of the cationic porphyrin is ascribed to the tautomerism of the inner N-D deuterons with the rate slower than that of the N-H protons. At lower concentrations (similar to 10(-5) M), both DiMPyP's form dimers in water in the presence of KNO3, while TMPyP(4) and TriMPyP exist as monomers. The association constants for dimerization in water at 25 degrees C have been determined to be 7.36 x 10(5) M-1 for 5,10-DiMPyP in the presence of 0.05 M KNO3 and 1.12 x 10(6) M-1 for 5,15-DiMPyP in the presence of 0.01 M KNO3. Large and negative Delta H and Delta S values for dimerization of these DiMPyP's suggest the London's dispersion force as the main binding force.