Cationic porphyrins are under study in a number of contexts including their interaction with biological targets, as possible therapeutic agents and as building blocks for molecular devices such as molecular photodiodes and solar cells. Many cationic porphyrins dimerize readily in aqueous solution. Dimerization in turn can control the properties of the porphyrin as well as its binding to its target. The propensity of a porphyrin to dimerize in aqueous solution can be estimated by recording the optical spectrum of the solution as a function of the concentration of added salt. Analysis of the data in terms of the Debye-Hückel formalism gives an estimate of the extent of dimerization as a function of ionic strength. Data for TMPyP4 [meso-tetrakis(4-N-methylpyridinium)porphyrin] and its butyl and octyl homologs; TMAP [meso-tetrakis(4-N,N,N-trimethylanilinium)porphyrin]; T theta PP [meso-tetrakis[4-N-[(3-(trimethyl-ammonio)propyl)oxy]phenyl]porphyrin] and the ferrocenyl porphyrin P3Fc are discussed. Dimerization may affect binding of the cationic porphyrins to their targets, e.g., DNA.